Depends on what you want to play, at what resolution, what your GPU is and what you want to spend!
Given that the graphics card is the most important element and Tom's has shown that for a mid-range card the CPU makes little difference, it's probably okay-ish for many recent games, for very little outlay. I have an Athlon X3 460 (just a little quicker than a 250) and that plays Metro2033 with good detail at 1080p and my 6850 is the limiting factor.
Of course a dual core Athlon isn't very future-proof, more and more games are utilizing a 3rd core and in a year or two you'll probably want a quad core.
Obviously if you spend more you will get a much more powerful processor.