You didn't specify the motherboard if he does have a PCI-e 2.0 or 3.0 slot then I recommend the GTX 670. It's a great graphics card with alot of power. It's right in your budget ( depending on where you buy it from, newegg has them from 399. ) They have 20 - 30 rebates with a free copy of botherlands 2. NCIX has them from 344. http://www.ncix.ca/products/?sku=71127&vpn=67NPH6DV5ZVX&manufacture=Galaxy%20Technology&promoid=1267
It really depends on where you look. But that card will have enough power for any modern game at high setting. On something like 1366x768.