For internet downloading and browsing the cpu isn't what will make any big difference there. Having a good internet connection and of course having dsl or cable beat dialup any day of the week. The only time the cpu plays any real role is when you are doing any online gaming to some degree there. That's when you see any good load on the cpu or when running multimedia.
For the basic system the AMD64 3500+ Socket 939 has worked out here even with some of the more demanding recent games out. For multitasking the better equipped AMD X2 line of models are a consideration. The other thing that will effect your decision is budget. What is the price range you are looking at for a new cpu?