You can use both the Radeon HD2600 Pro or GeForce 8500GT, though they're not very good for playing the latest games. If you stretch your budget a little, you could get a GeForce 9600GT which would give more than twice the performance of either card. I'd recommend going for the AthlonX2 6000+, it's much cheaper than the FX-62 and offers similar performance. The ASUS M2N should support it without a BIOS update. Go here for info on choosing a quality 650W+ PSU:
Power Supply Information and Selection