This CPU absolutely screams, and is only $60. But it is AM2, so you would need a new mobo too. But it's really an amazing CPU for the price.
It's priced so low because it's single core, but it's the 2nd fastest single-core CPU on the market right now as far as I know.
I've got one in my current computer, and I can run a 140-dwarf fortress at about 25-30 fps steady, which I'm pretty sure is pretty good. I've got 2gigs of RAM as well and a nice AM2 mobo (which was only ~$70), and this computer is faster than any other I've ever used.
The secret to making a very fast computer for cheap is going to Newegg, sorting each component by best rating, and buying the components with 5/5 stars at the top of the list which are inexpensive. Take a look at the reviews for these components and you'll see that people uniformly are both very happy with them, and that they are surprised at their extremely good performance and quality, "considering the price".
Price and many of the supposed things that make a computer fast aren't really worth judging components on. Sure, that supposedly godly CPU is dual-core or quad-core, but for instance this CPU is $50, and it gives amazing performance, and can be OC'd to 3.0ghz with no problems.
If you're going to upgrade to another CPU and you want to do it for less money, get an AM2 mobo and plug this puppy in, you will be very happy.
But also you can get a 1gig stick of RAM for about $50 now too, which would help your setup tremendously I think. But I love this CPU so I figured I'd chime in a bit about it.
[ February 23, 2008: Message edited by: Old School Gamer ]