Hmm...depends what processor you are looking for. An AMD Athlon 64 is of course better for games than an Intel pentium 4, which costs more too.
You might be best getting a 939 pin Athon 64 (will hold you for the future too) like the 3000+ or 3200+. These aren't that expensive, and will perform just as good as a Pentium 4 running at 3.0ghz, and 3.2ghz, even though the Athlon's run at 2.8ghz for the 3000+ and 3.0ghz for the 3.2
Since Athlon 64's are cheap though (you might have to buy a new motherboard) I think this would be the best option, since you could get a fast system for much cheaper than you could get a Pentium.
I hope this helps