the 6600 is a greatly priced card... but honestly dude... try it out... it doesn't look bad at ALL even on lowest settings...
My systems aren't much beefier than yours... but with my slightly beefier video card I pushed half life 2 to max settings without problems. I pushed doom3/quake pretty high up also
P4 2.8 and AMD 3200+
1gb ram corsair XMS both
Radeon 9700 Pro, radeon 9250 pro (i switch and swap lol)

but seriously ur system isnt bad at all... just get a video card and you can run top notch shit.... but honestly ... (well unless you can justify spending 100bux on 1 game...) low-detail mode doesnt look bad at ALLL.. I think "net works" at discover mills runs 5200's... either that or 5700's.... dont remember

but check it out... the low detail mode doesn't look bad at all. If it were an ATI card... turning up AA and AF gives more clarity on BF2 and HL2 and also keeps the framerate up... but for 100 bux the 6600 is the way to go... mate that with a zalman copper fan and you're set you can do some major overclocking on the 6600