2gig - 2.4gig won't make as much difference as the video card.
Go for the 2Gig if you want to save money.
Over at Tomshardware.com is a comparison of CPU speed compared to Video Cards and the 9700 Radeon is faster on slower processors than the GF4 series.
Sound cards aren't too picky, some use more CPU than others. Just get a reasonable one, SBLive's are cheap and work well enough, get the 5.1 DE if you want to use it for outputing to a stereo/home theater setup.
Onboard sound will be fine for just about everything, only thing a SBLive etc is used for is 3D sound in First Person Shooter games.