Paw
It's easy. If Sonar see your RME midi ports, then go to add track - instrument track - and choose TTS. Then in input signal choose your RME midi channel ( omni it's good for start ), and you will hear your TTS sound when you play on controller. Rest is for you, you can record, or change sound in TTS.

I must be missing something.....Where would I hear this TTS sound coming from though?  I use a MIDI keyboard controller (no sounds).  

Midi make no sound itself, but if you connect your midi output from controller to midi input in softsynth like TTS in Sonar, then you'll hear your track. This is the way that we all go, midi composers😊.
What version Sonar you have?

Oh, so TTS is just another synth?  I've already got a synth that can load soundfonts just fine.  That was never the problem, the issue was latency with that approach.  Which is why I relied on the SBLive card.
 
I'll try experimenting more with latency on the new DAW rebuild, just don't care for the idea of having to crank it down low enough to feel real-time as I suspect once I get around to running two audio dozen tracks and plugs that it will probably start tripping.  I like to set the Sonar config once and leave it alone, really don't want to get into a scenario where I'm having to remember to readjust latency based on whether or not I'm trying to input Midi tracks or rather audio tracking/mixing.   Those are the implications of what I foresee at the moment unless I'm misunderstanding things....
 
Sonic

You where experiencing latency most likely because your not setting things up properly and/or using the wrong drivers. RME is as good as it gets and you defiantly need to be using that as your DAW's audio interface and drivers.
 
You may even need to remove the Sound Blaster if it is a PCI card because they are well know to steal the drivers for other devices cause more trouble that they are worth. Creative drivers ( in the past) are notoriously bad and do not run smooth with Sonar. I know first hand as I have a Creative Audigy II PCI card. That was why I ended up on this forum in 2004, I couldn't get anything to work in Sonar. Things improved for me immediately when I switched to a proper USB interface. I installed the Audigy in one of my DAW set ups once and ended up having to remove it because it conflicted.
 
So make sure in preferences that the RME is your audio driver and clock source.
Make sure your using ASIO drivers and best if they are up to date.
 
Make sure your studio monitors are using the 1/2 output of the RME. You DO NOT want to use your computers output.
There is no reason that you should not hear a VST instrument and with zero latency.
 
There's a few other trouble spots.
 You PC is grossly outdated, you mention a re build??
Might be a good idea to post the complete system specs including RAM and which OS your running.
And which version of Sonar are you running on that?   
 
Also your RME is firewire, there are well know issues with having the proper Texas Instrument chipset when using Firewire.
 
 

The RME is as good as it gets so if you've got latency issues, its because of something else. If you're having latency issues I would suspect the soundblaster and use of soundfonts.
 
Forget soundfonts. Use the midi input on the RME and drive soft synth and/or soft sampler plugins right in Sonar. I do this routinely with absolutely no latency issues. I'd also consider pulling the soundblaster out of the machine. Simpler the hardware/install the better when it comes to DAW's and latency.

I'm using WDM/Kernel drivers, not ASIO (although frankly I can't recall why).   The SBLive is not in the rebuild, yet.  That's what I'm now trying to figure out.
 
System is a dual core Pentium 3GHz, DDR2, XP.  So while not exactly "modern"it has plenty of horsepower.  More than enough for my needs.   And yes, using TI chipset on the Firewire card, never had a problem with the card or the RME.  Although I am probably going to try a PCIe card (also TI chipset) this time around.

I will experiment with the latency settings....

Yes, you should use ASIO driver, RME have best drivers I know.

You still din't say what your RAM is and what version of Sonar. 
You are running a very outdated system and things have changed, Hard to trouble shoot with half the information. 
Start with switching to ASIO and trying a VST instrument and report back.