You should go AMD if you're not going to go with Core 2. The only worthwhile offering from Intel right now is Conroe.
For $100 you could go with an Athlon or Opty. Whether you want single or dual core is really up to you, but I personally would recommend dual core.
The Opteron 148 is a very good single core processor for $106.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103596
It has excellent overclocking capability, and even if you leave it at stock speed, it should still be fast enough.
Unfortunately, it's socket 939. I don't know if you're willing to go with 939 or not. If you're going with AM2, you're going to want something like the Athlon 3200+ (there aren't many AM2's for under $100), which is a good processor, but the Opty is faster, more reliable, and will overclock better. It depends on how willing you are to buy another motherboard next time you want another processor upgrade. You'll need one if you go with 939, but with AM2 you might not.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103637
If you want to venture into dual core, the 939 3800+ X2 is only $108 right now. It's an excellent deal, and I think that's ultimately your best bet.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103531
Again, though, that's socket 939, so take that into consideration regarding future upgrades.
Unfortunately, the AM2 3800+ X2 is a good $61 more expensive.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103735
EDIT: Ok, I was accidentally looking at the single core 3800+ instead of the dual core X2. Sorry for that mistake. The 939 X2 3800+ isn't actually on sale at NewEgg right now. AMD discontinued the manufacturing of them, so I guess they're finally running out. I'm sure you could find one somewhere else, though, but it'll probably be around $150, so that kind of invalidates my point about the 939 vs AM2 pricing. $108 did seem rather cheap...