That's some nice stuff, but I don't really see how you will get an 'awesome performance difference' from it, no offense.
The XP 2500 has an unlocked multi right? Make sure to get the most out of that RAM by lowering the multi if you have to. You should be able to go 250x9 or somewhere around there and have about the same CPU speed but increased bandwidth.
Was your PSU a limiting factor in your OC'ing ability? If so, then maybe yeah, you'll be able to get a higher CPU clock as well.
I guess you'll be pushing the 6600GT further as well, so hey maybe it will be a pretty decent upgrade lol. Congrats