I have a MOTU 828mk3 hybrid for my studio computer, but occasionally I open Sonar X1 and edit projects on my office computer, which is just a desktop AMD 955be with the onboard realtek audio.  I would like to get a cheap audio card for it (either firewire or USB) that is very basic, Asio drivers, 2 audio outs and 2 mic/inst inputs.  I was looking at M-Audio Mobilepre or the Fast Track.

Would either of these suffice for some occasional edit work (primarily playback but occasional audio/midi input)?  Is the latency horrible on these cards?

Is there another cheap asio card that would work for an office/desktop computer?  Again, I already have a MOTU in the studio, this is just for my office computer for fiddling around and editing Sonar X1/2,

I've had a Fast Track as part of my mobile set up for the last 2 years. It has served me well. I can get the buffers down to 32, so latency is pretty low and it has a direct monitoring option. I've tracked guitars and even some vocal demos w/ it. For your purpose, I think it'd be more than adequate. 

No issues whatsoever w/ it, the only reason I have to replace it is that we're finally settling down. The 2 RCA outputs on it just won't cut it for me any longer and I'm looking for something w/ more options, more I/O's and slightly better converters if possible. 

Probably one the cheapest option I've seen was the Lexicon Alpha which seems to do exactly what you want for $60 or so. I have no experience w/ that one though.

Thanks Rain.  It must be acceptable if you recommend it.  Just to be certain, I am referring to this one:   http://www.m-audio.com/products/en_us/FastTrack.html .  I know the pro and ultra versions are good, but want to make sure you are referring to the same one , the basic one and not the pro or ultra.
Thanks

Yup. That's the one. If you don't have to rely exclusively on the average preamp and can work w/ the RCA outputs, it's absolutely fine.  :)

I'm using a Focusrite Saffire 6USB for my "downstairs" rig and it works perfectly - and of course you get 2 F/R pre-amps, which is something they've always been fairly good at!

Roland is coming out with a Duo Capture EX. It has what you want. There is a Tri-Capture that may be what you want. There is a Duo-Capture out but it does not have the XLR input.

It all depends on how important the Pre amps are.
For just editing you don't even need pre amps. There are a few DJ style Audio interfaces that are under a $100 that only have RCA in/Out that have good stable drivers. ( Not Behringer) 
 
My son uses a Native Instrument box he paid $75 without issue.
 
I have this PCI card in my office computer that uses ASIO drivers and runs Sonar just fine. Can't remember the brand but it was made for Radio station workstations I got it free. Has 1/4" TRS in/out. 

If you do need pre amps, I don't recommend M Audio, they are almost unusable.  And Lexicon comes up with a lot of driver issues if you look around the forums.