Its pretty easy. Builduing a PC is like Lego Advance. Power Supply everyone should work and VideoCards also.
You could buy a 500 Watt Power supply (cheap one I assume ?): Here some examples http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=500+watt+power+supply&x=0&y=20 ranging from 20-60. I personally would go for a 500 Watt.
Installing it is easy just loose the screws loose cables take out slide new one in screw it tight and put the cables in thats it. Here a better text: http://www.pchardwarehelp.com/guides/Upgrade-Power-Supply-Guide.php
Installing Graphics card mechanically is simple only the Software side can be a bit annoying. Also maybe he will use the Integrated graphics instead of the graphics card so you have to turn it off in bios.
Take the 6450 with 1gb DDR3 it will work also has HDMI/DVI which is always good. It will give your PC a substantial boost. Graphics will look MUCH better then they do now. To make it simple for you I would take these components:
http://www.amazon.com/Sapphire-Radeon-PCI-Express-Graphics-100322L/dp/B004XLPJNK/ref=sr_1_2?ie=UTF8&s=electronics&qid=1314683333&sr=8-2
and this:
http://www.amazon.com/Diablotek-500-Watt-Power-Supply-PSDA500/dp/B001P92AA8/ref=sr_1_1?ie=UTF8&s=electronics&qid=1314684095&sr=8-1
Total 75 Dollar
For Installation of graphicscard watch this please: http://www.youtube.com/watch?v=O9x097QRXeA
How to install Power Supply watch: http://www.youtube.com/watch?v=s0udZ0Ky4Pk
Both things I listed work on your computer if you encounter problems while installing or are not sure about something there are already tons of threads out there that solved your problem just google it.







