Your power supply will be too wimpy. Your old OS won't prob have drivers for the new video card. For an extra couple hundred you can get a whole new computer. Here's a Cyberpower I threw together.
$888.00 (before all applicable rebates)
CASE: ($20 off Mail-in Rebate) RaidMax Smilodon Med-Tower 420W Case w/ Side-Panel Window CPU: (Sckt775)Intel® Core™ 2 Duo E8400 CPU @ 3.0GHz 1333FSB 6MB L2 Cache 64-bit MOTHERBOARD: MSI P7N SLI-FI 750i SLI Chipset LGA775 Supports LGA775 FSB1333 DDR2/800 Mainboard w/GbLAN, USB2.0, &7.1Audio MEMORY: (Req.DDR2 MainBoard)4GB (4x1GB) PC6400 DDR2/800 Dual Channel Memory (Corsair or Major Brand) VIDEO CARD: NVIDIA GeForce 9500 GT 1GB 16X PCI Express (Major Brand Powered by NVIDIA) VIDEO CARD 2: NONE VIDEO CARD 3: NONE LCD Monitor: NONE HARD DRIVE: Single Hard Drive (500GB SATA-II 3.0Gb/s 16MB Cache 7200RPM HDD) Data Hard Drive: NONE Optical Drive: (Special Price) LG 20X DVD±R/±RW + CD-R/RW DRIVE DUAL LAYER (BLACK COLOR) Optical Drive 2: NONE SOUND: HIGH DEFINITION ON-BOARD 7.1 AUDIO
If you can upgrade the video card, I'd look at a GTX. You may also need a larger PS than 420 watts, but they'll tell you if you do. 600 watts is good if you like games and want a decent video card.