Nice first post! Plenty of info, thats how it should be done.
There are quite a few VoodooHDA for Snow, try a different one in case you havent yet,
this one from Slice seems to be actively developed and patched.
What kind of graphics flickering do you get? I get some small brightness flickering, specially noticeable on grays, witha GF 8600GTS (same core as yours I think) but nothing very noticeable. I got no fix for that but it doesnt really bother me.
With USB slots do you mean some controllers not being detected in System Profiler or something else? Try playing with Legacy USB Support in the BIOS if you have that option.
The Realtek kexts have troubles in 64 bits, you should try running Snow in 32 bits, it would make things much easier, once you get everything working in 32 you may consider switching to 32 bits. In general I dont see the point in running in 64 bits unless you have very large amounts of RAM but well thats just my opinion.