gaming gfx card hierarchy chart:http://www.tomshardware.com/reviews/fastest-graphics-card-radeon-geforce,3085-7.html
your psu seems iffy.. never heard of optimax, can't find any review/specs...
your motherboard seems to be able to support up to phenom ii with a bios upgrade, check your motherboard's cpu support list in case you want to upgrade your cpu.
be able to run using a 500+ watt psu..
biggest factor is how many amps your psu can continuously supply on its +12v rail(s). cheap psus usually report only max. figures, not continuous figures.
with a psu upgrade, you could go up to radeon hd 6850 depending on budget, gaming resolution, other specs.