You'll be hugely bottlenecked on an Athlon II X4 640. It's way slower than the Phenom II that's destroyed by the 2500 in this test:
http://www.xbitlabs.com/articles/cpu/display/core-i5-2500-2400-2300_7.html#sect0.
And while some people will be tempted to say "oh, you'll get perfectly good framerates on an Athlon, your minimums will just be a little lower," take a look at the numbers for Starcraft, RUSE, and Metro 2033. Not a chance. The i5/i7 kills AMD in modern games.