Here's a link to the CPU support page over at Asus:http://support.asus.com/cpusupport/cpusupport.aspx?SLanguage=en-us
Looks like you can go up to AM2 6000+ CPU, although I think those are hard to find in the retail channell. You could try ebay, but they tend to be a little overpriced in my opinion.
For the CPU, I would look at the 5000+ CPU or 5600+ which are easier to find for around $50.
It would be helpful to know what else you are running in this PC, for instance a faster CPU won't do much for load times, those are more dependent on your ram and Hard drive. This would make it easier to determine if you need a more serious overhaul. Ultimately though, AM2 is a pretty dead end socket and before dropping too much money on this older hardware I would consider your needs moving forward.
I think you'll see quite a boost in performance for the CPU upgrade however, probably worth $50.
I'm guessing your ram is ready to be upgraded.