What you Should do is get a new GFX card!
but your PSU sucks ass because I would say get a 9800gtx2 (2 - 2 GPU's 1gig on card) there the best bang for $$$ and they overclock really nicely
"just keep them cool"
But with your PSU I would say go buy a nvida EVGA 9500gt off of NEWEGG.com
or you could get a EVGA 8400gs or a 8500gt but that's really all your PSU can take.
thats why buying PC like DELL HP and so on are a big wast of $$$$$
(and there made like shit and you can't over clock them without Flashing the BIOS)
for the $600 you hp cost I could of made you a way better PC
Athlon 2 x4 3.8GHZ
GFX Card 9500gtx "super clock" 1gig on card
motherboard ASUS
tower Coolmaster
PSU 500w
4-5 gigs of DDR2 RAM
Hard Drive Western Digital Caviar Black 500GB 7200 RPM 32MB Cache SATA
( thats with out keyboard, mouse, and screen)
And I would cost about $600 - or less " I get 50% off on every thing at newegg because I buy a lot of shit there" LOL
I just made one for a friend 2 weeks ago