That PSU is a bit weak with only 33AMPS combined on the 12V rail, but since your on a build i assume its ok. Whats the resolution you want to do the light gaming on? If you get the 8400GS its not really ment for gaming at all. I have one and playing COD4 at 1152 x 864 its a bit laggy lol. Its more for Blu- ray or HD. You could maybe upgrade it to a 8600GT atleast if you want to do gaming. RAM is very important when running vista, i recomment atleast 2GB of DDR2 ram, because Vista is very reource demanding. If you've any more questions feel free to ask.
It's going to likely be my little siblings playing the game and they won't spot the difference in resolutions. Good call on the 2gb, I'm probably going to go with that. Any recommendations for another PSU?
Antec NSK4480B Black 0.8mm cold-rolled steel construction ATX Mid Tower Computer Case 380W Power Supply - Retail
$99.99 -$20.00 Instant $79.99
A-DATA 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model ADQVE1B16K - Retail
LITE-ON 22X DVD Burner Black SATA Model iHAS322-08 - Retail
Microsoft Windows Vista Home Premium SP1 64-bit for System Builders - OEM
Add the floppy and the wireless if you need them; it won't add much to the price.
Another option would be to go AMD on a 790GX board, and forego a separate GPU for now. The HD3300 IGP is capable of light gaming. That build would be around $450, including the OS but not those other two things. Neither include a keyboard or mouse, but then neither did yours, so I figure they're not needed.