You'll need to check how much you can install, as there's no point buying loads fo RAM if your system can't take it. You'll also need to post the motherboard make and model so we can see the maximum processor speed as well.
Socket 754 CPUs are getting more scarce as they're a bit old now, but there's a good price one here as an example.
-------------------- OK OK I can't go any faster! aaaaaaaaaaaagh!