Registry cleaner, installing XP every three months, I always bang my head on the wall if I hear this. If people only learned how to make use of something like
http://www.terabyteunlimited.com/image-for-dos.htm they would always have a clean system.
Concerning hardware, I think the ideal price performance spot is a Core 2 E8400 and ATI 4500, at least that was what I chose after a long search.
Or you can wait for the Nehalem architecture to materialize, it should give a nice performance boost with its integrated memory controller.