Well, it has a 300W PSU. That will be the limiting factor as it sits right now. The PSU will have to be replaced in order to use any graphics card of consequence.
The Athlon II X4 630 is not a slouch of a CPU by any means. It will handle anything he throws at it.
It has 64-bit Win7 and 6GB of RAM, as well as a 1TB hard drive.
My advice would be to grab a 550+ watt PSU, like this:
Newegg.com - BFG Tech GS-550 550W ATX12V V2.2 / EPS12V V2.8 SLI Ready CrossFire Ready Power Supply - Power Supplies
As for the GFX card, it will depend on how deep his pockets are. The 5770 runs around $160-$170 for a XFX card (lifetime warranty), while the 5670 runs around $100-$110 for the XFX.