Well AMD's do much more per clock cycle than Intels so a 2.4Ghz FX-55 64bit CPU will outperform up to 3.6GHz Intel processor depending on the model.
PC2100 is incredibly slow especially if you are planning on an AMD64 system you basically need to get rid of that.
If you want to go the 64bit CPU route I suggest something like the ABit AX8-3rd Eye( this board is PCI-Express so if you want AGP you'll need the AV8-3rd eye) or the MSI Neo2-Platinum(AGP) both of which I found to be good boards. IMO asus has gone down the hill in quality and tech support REALLY sucks bad...but a lot of people still like them.
Those boards I listed are like $125-$150 and a good Socket 939 CPU would be something like the 3500+ AMD64 which I believe is down to like $150 now.
There's already $300 and you still need to get no less than 512mb of RAM preferably 1gb if you can fork out that cash for some PC3200 DDR400 RAM.
You don't really need high performance RAM unless you're overclocking so if you're not then I'd suggest just sticking with some good ValueRAM by one of these companies:
Those are all good RAM makers IMO.
Best websites for purchases are:
incase you needed some resources.
I suggested the 3500+ because below that performance wise it's somewhat worthless, and going above that is generally pretty pricey CPU's