thing is the GHz as a faster clocked single CPU should work faster that a slower clocked duo CPU in games. Especially since most games aren't written for dual core processing.
However, you'll find your general work life quicker with a dual core CPU, as you can do multiple things within windows without the computer lagging too much. And on performance tests the dual core CPUs do seem to be generally faster.
In any case, single core athlons (I think) are only available on Socket 939 motherboards which is obsolete now. So if you're going to go for AM2 the go dual core.