Umm, .3ghz isnt a much of a difference processing wise, although depends.
You might notice a bit of a difference if you had a single core processor @ 2.7GHZ, compared to a 3GhHZ one..
Maybe not as much on a decent quad core, but not much difference either way..
How much is 'cheaper the better' R.E. a CPU?
$97 here will probably be enough to get a low game of crisis:
(I think Minimum specs are equivalent to a P4 @ 2.8GHZ off the top of my head)
Personally I dont like spending less on a CPU than you have done on a processor, so for around $140 ish perhaps a Intel i3, or if you could afford an i5 (3.4GHZ ish Dual core) that would do the trick.
Two things to note -
1) You will probably get a little better performance for your $$$ (if you still want relatively cheap) by going AMD, and getting a Phenom II x 4 955 or something??
2) Are you willing to Overclock at all? Might be something to consider if you wanted to / know how to.