I´d also say: save some more money to get a P4 board with AGP slot and a nice 6600 (non GT) or Ati 9600pro maybe.
Would cost not more than 200$ prolly and the performance gain will be HUGE!
If you reinstalled Windows (as you said) I think the problem may lay there.
Since you changed nothing at the hardware (well, try your old RAM stick only to see what happens) the chance is quite big that your software installation is a bit faulty.
Install all the latest drivers, especially chipset.
Cause a new stick of RAM won´t cause the system run slower....usually....
the links are not working here, prolly my DNS doesn´t know dabs.com ^^
Maybe post the names of the CPUs pls?
Edit: nevermind, I added another DNS server to my list, it´s working now ^^
OK, I would say take the 3700+ because it has the bigger cache.
If you want you can overclock it afterwards, and it will always be faster at the same cock speed than the 3500+.