The 320mb version is good for lower resolutions like 1280x1024. If you have 1600x1050 or above id get the 640mb version.
And why not go with the 8800gts? In my opinion it is good both ways; its not as expensive as the 8800gtx but it still packs a punch and is dx10 supported.
The lower end models like the 8600gt (is it gt i forgot?) might not be out for another month (or more) unless a release date has come out i missed.
CPU: e6600 @ 3.2ghz
RAM: 2GB G.Skill HZ PC6400
Mobo: ASUS P5B Deluxe
GPU: Sapphire HD4870
Soundcard: Creative Labs X-Fi XtremeGamer Fatal1ty
HDDs: 250GB WD + 750GB WD + 2x1TB WD
Monitor: Samsung 226BW
PSU: OCZ GameXStream 600W
Case: Cooler Master Stacker
OS: XP Professional SP3
Mouse/keyboard: Logitech MX518/Saitek Eclipse II
Speakers: Logitech Z-2300