Help upgrading x800 card on old barebones

Hey guys i got a barebones pc some years ago and had the x800GT radeon card in it. My system is old so starting upgrading it purely to play things like World of warcraft on better than extremely low settings. Also if possible ready for guild wars 2 and the old republic. Or some first person shooters. Or at least some good graphic games, but nothing like crysis as i know my pc is terrible and old, try not to laugh at it =p.

My Pc:
Asus Vintage Ah1 Barebones
AMD athlon 64 3500+
4gb Ram
400w PSU - octigen
64 bit windows 7 ulti
x800gt radeon

I'm looking to get an x2 4800 or 4600 processor from ebay etc (939). I recently upgraded the ram from 1.5gb to 4 and its so much faster switching from games to browsers etc. Some websites are very horrible to load video it stops and starts etc and is jumpy. Net is very fast so i think its deffo the hardware.

I just need some advice one what i could get maybe with the 3500 processer or when i upgrade it to an x2. i have a budget of around £50 and just want to be able to play some decent games without taking a year to load.

I've been getting a little confused with the compatibility of pci express 1.00 and 2.00 etc and the fact some cards may need another power source aside from the bus power and i've been scowering the internet for ages to make heads or tails of it, so any recommendations and help would be greatly appreciated! Thanks very much