Jim Roseberry
If your audio interface has high round-trip latency, there's nothing you can do about it (short of doubling the sample-rate).

Although unfortunately, with some interfaces, doubling the sampling rate also results in doubling the buffer size (to compensate), in which event the overall latency will still remain basically the same (although the ADC and DAC conversion latencies may be decreased, they're typically quite small in comparison).
 
Also, it is important to keep in mind that as buffer size is decreased (to reduce latency) a greater load will be placed upon the system (which needs to work harder to service the buffers more frequently). So not only interface latency but also system loading may need to be considered.
 
Info on which audio interfaces are able to offer good low-latency performance when running softsynths may be found here (see DAWbench VI (CV and NCV) test results):
 
http://forum.dawbench.com/showthread.php?1548-Audio-Interface-Low-Latency-Performance-Data-Base&p=16069#post16069
 
http://dawbench.com/audio-int-lowlatency2.htm

Goddard
 
 
Although unfortunately, with some interfaces, doubling the sampling rate also results in doubling the buffer size (to compensate), in which event the overall latency will still remain basically the same (although the ADC and DAC conversion latencies may be decreased, they're typically quite small in comparison).
 
Also, it is important to keep in mind that as buffer size is decreased (to reduce latency) a greater load will be placed upon the system (which needs to work harder to service the buffers more frequently). So not only interface latency but also system loading may need to be considered.
 

All true...
 
ie: MOTU and RME units double the buffer size when you move up beyond 48k.
But... these units provide low round-trip latency... so (in this case) it's moot point.
 
Doubling the sample-rate will significantly increase CPU load.
If you're running a fast DAW (four or six CPU cores at 4.5GHz), then the increased CPU load may be worth it (If that's your only option for workable RTL)
IMO, It's best just to start with a unit that offers low round-trip latency.  That way, it's never an obstacle.   
 
Most all PCI/e audio interfaces yield 5ms or less total round-trip latency at a 64-sample ASIO buffer size/44.1k
The Presonus Audio Box VSL series yields 4.9ms total round-trip latency at a 64-sample ASIO buffer size/44.1k
RME USB units offer 4.9ms total round-trip latency at a 48-sample ASIO buffer size/44.1k
MOTU USB/Firewire units yield 5.5-6.5ms total round-trip latency at a 64-sample ASIO buffer size/44.1k
M-Audio FastTrack Ultra/8R units yield 5ms total round-trip latency at a 64-sample ASIO buffer size/44.1k (using High Performance mode - reinstated with latest driver)
Steinberg MR816 units yield 5.5ms total round-trip latency at a 32-sample ASIO buffer size/44.1k
Focusrite units yield ~6.5ms total round-trip latency at a 64-sample ASIO buffer size/44.1k
Roland "Capture" units yield 7.4ms total round-trip latency at a 48-sample ASIO buffer size
 
Though inexpensive, the Tascam USB units are some of the worst performers when it comes to (low) round-trip latency.

First of all, thanks so much for all the people who replied here; Jim, of course, in particular, with his very detailed response.
 
I eventually decided to give the Scarlett 6i6 a try (and have posted brief comments in a separate post). For the price point it offered me four analogue ins and four outs, all balanced (afaik), two of the inputs will handle mic/inst, which is all I really need.
 
The Centrance latency test indicates a 6.4ms round trip latency at minimum buffer size. Reaper indicates a 3.3ms latency one way which is pretty consistent with that measurement and at that setting I've tried both three instances of Kontakt playing large sample sets and the 'ReaperTrek' project with around 18 MIDI tracks feeding into a single instance of Garritan Personal Orchestra (ARIA player) and can see no evidence of any dropouts or glitches with CPU sitting around a few percent (core i7 Acer laptop).
 
So at present given the cost differential between this and the RME products (and I did not much care for the Babyface breakout cable approach), I am very pleased with it. It is also - hooray - not bus powered. I've always thought this was a profoundly stupid thing to do because it just about guarantees ground loop problems due to the very spiky current draw down the USB cable causing a very low impedance noise source that can be very difficult to eliminate. I mostly got rid of the problem with the old UA25 by using some very heavy gauge copper cable (insulated cable with a very flexible copper braid conductor which is sold for wiring up car audio systems) and attaching this to the main mixer as a bonded ground connection back to the UA25, with a spade lug attached to one of the rear panel screws on the UA25, but it was still necessary to use ground-lifted cables between the mixer and the UA25 and with the Focusrite all these problems appear to have vanished - fingers crossed.
 
Incidentally I did look briefly at the larger 18i8, which does offer four mic preamps. I don't really need that but it is quite cost-competitive. However you only get two line outs on the back panel and I wasn't quite sure how the two front headphone outs worked in relation to the other outs - on the 6i6 they monitor line ins 1/2 and 3/4 respectively, so it seemed like you actually got less outs on the larger unit. And it is a good deal larger, physically; not an issue in the studio of course but I was thinking about potentially gigging with the laptop, where the smaller 6i6 would be easier to handle. In any case, I figured if I really needed more I/O then the (non-optical) SPDIF in and out on the 6i6 could easily be wired into another convertor to add another stereo in/out set.

Yes the Tascam runs 10 - 12 ms @ 44.1 on  my system. I only recommend these units to people looking for a lot of input power for live recording. Good value for that, but not a good performer for real time efxs at all. Someday I wish to use a laptop for my efxs unit and I too will be asking this question, Thanks for your post Jim, that is very valuable info. Found it interesting the RTL spec on the Roland which seems popular round here. 
 
I also always recommend you look for the AC power feature. I have had issues with bus power not supplying enough Phantom Power through a snake.
 
Also pay attention to the monitoring set up. Some units are badly implemented for monitoring.