Compatible Hardware for HP a420n

You should be able to use any AGP video card and a PSU around 300 or 400 watts. Corsair is a good PSU brand. This is one of the best AGP video cards you'll find: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
It'll work for basic CAD but don't expect high-end gaming too.

... but really, it's not worth upgrading this system. You can't add any more RAM according to HP, and a graphics card is probably a waste of money. It doesn't even have USB 2.0. A $300 netbook will probably run just as fast.