All sounds fine to me.
This is very similar to the spec I run (apart from my CPU is an AMD 4200+ X2 and my RAM is standard DDR).
I'd definitly suggest the 8800GT over the GTS (much better bang for your buck).
As to the motherboard - Most good quality boards which are any good for gaming come with SLI or Crossfire anyway (you don't have to use it after all), but I would always recommend ASUS. I've had a couple (amongst many others) and they are excellent performers and very reliable. I'd suggest one of the nvidia nForce based chipset boards, such as the P5N32-E SLI which is basically the intel CPU version the motherboard I have.
My spec is as follows and runs Crysis fine in 1280x1024 all on ultra high quality (used a small hack to enable the so called 'DX10 only' graphical features).
CPU: AMD Athlon X2 4200+
RAM: 2GB DDR 3200 (in 512MB pairs)
HDD: 2 x 250GB 7200rpm Maxtor
MoBo: ASUS A8N32 SLI
PSU: OCZ 600W StealthExtreme
(And a bunch of other gubbins)
The only thing I'd suggest is that you make sure that you PSU is upto the job - 600W is easily enough (if that is what it actually provides!), but its all about the stability and oomph of the 12V rail(s) when dealing with powerful gfx cards. I'd always suggest a PSU with more than 1 12V rail (ideally 3 or 4) with at least 15+ Amps per rail.
Hope that helped.