A single 7900GT will run just as fast in many (most? all?) games as an SLI 7600 setup.
4 gig ram... Is that 4 1 gig sticks? If so, you'll probably lose the ability to run the memory at 1T command rate timings and will have to use 2T for a slight loss of speed. Maximum 2 sticks of memory for AMD = teh faster.
Others have pointed out the possibility of problems running AH on dual core AMD cpus. YMMV, but a faster clock speed single core cpu will run AH faster than a slower clocked dual core cpu.
No sound card? You may lose a few percentage performance using onboard sound. Since you're putting premium money into this however, I'd suggest a few changes.
Ditch the SLI setup and get a faster single vid card. 7900GT or GTX.
Ditch the dual core cpu and get the fastest single core cpu you can afford.
Ditch the 4 gig ram, go for 2x1 gig sticks, and spend the saved money on one or both of the following items.
1. Soundblaster x-fi soundcard
2. Latest Western Digital Raptor 160 gb