i.e SATA drive? that much better?
They are at their best when you need more than one hard drive, because, unlike IDE HDs, they have their own dedicated channel.
RIght now? No. Later on when one 6600GT can't play the latest game, yes. By the time that 1 6600GT cant play a game at a decent frame rate, the cost of getting a second 6600GT will be pretty low. That is why I recommend the SLI boards. If you dont mind just upgrading the vid card and not just adding in a second one, maybe go for a mobo like this:
ASUSnForce4 Chipset Motherboard For AMD Socket 939 CPU, Model "A8N-E" -RETAIL $138
This board is only $30 cheaper.
As for that RAM you picked, that is a little excessive. Just go with one of the budget types from Crucial, Corsair, or Kingston, etc. I still stand by my recommendation of the Kingston Valueram for $90. Without overclocking, your ram will be running at PC3200 speed anyways, so there is really no point in getting PC3700 RAM if you are not going to overclock.
If you someday went to add a 2nd vid card onto your SLI mobo, then yes. If you go with a non-SLI board, then about 450W will do. This might be a little more than is absolutely necessary, but the PSU is one thing you cannot be skimpy with.
As for a sound card, both this^^^^ mobo here or the SLI one come with very good onboard sound, and you shouldnt have to worry about getting anything else for a sound card.