Sub $400 Build Advice

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?

If you really wanted to do upgrades in the future or change that 8400GS to a better card something like this will do, http://www.newegg.com/Product/Product.aspx?Item=N82E168...
But if your just keeping this build for a long time then don't upgrade the PSU. Also you might want to check out a better CPU because vista is such a resource hog. Something atleast like a Pentium Dual Core CPU, this is a E2200 Pentium D CPU with 2.2Ghz and a 1MB Cache. http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Just a few suggestions

for $21 this 2gb DDR2-800 ram should run vista just fine
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Do you really need a floppy drive?

For $20 more, this 2.5ghz pentium dual core would offer much better performance in vista
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

For $12 more, the 9400gt can handle games and vista graphics quite a bit better than that 8400gs (but it wont run crysis)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I'm a bit uneasy about the quality of that psu as well.
Better ones do tend to be more expensive though, for $40 after rebate, corsair offers a very reliable, efficient 400W psu.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If you can stretch out that budget just a little, the following build will be vastly superior:

Intel Pentium E5200 Wolfdale 2.5GHz LGA 775 65W Dual-Core Processor Model BX80571E5200 - Retail
Model #:BX80571E5200
Item #:N82E16819116072
$69.99 $69.99

Western Digital Caviar SE WD3200AAJS 320GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Model #:WD3200AAJS
Item #:N82E16822136098
$49.99 $49.99

SAPPHIRE 100255DDR4 Radeon HD 4670 512MB 128-bit GDDR4 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Retail
Model #:100255DDR4
Item #:N82E16814102818
$79.99 $79.99

GIGABYTE GA-G31M-ES2L LGA 775 Intel G31 Micro ATX Intel Motherboard - Retail
Model #:GA-G31M-ES2L
Item #:N82E16813128357
$52.99 $52.99

Antec NSK4480B Black 0.8mm cold-rolled steel construction ATX Mid Tower Computer Case 380W Power Supply - Retail
Model #:NSK4480B
Item #:N82E16811129032
$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
Model #:ADQVE1B16K
Item #:N82E16820211188
$39.99 $39.99

LITE-ON 22X DVD Burner Black SATA Model iHAS322-08 - Retail
Model #:iHAS322-08
Item #:N82E16827106286
$25.49 $25.49

Microsoft Windows Vista Home Premium SP1 64-bit for System Builders - OEM
Model #:66I-01939
Item #:N82E16832116488
$99.99 $99.99

Subtotal: $498.42

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.