You can't really compare CPU speeds across different lines of CPU, only directly with each other. So, a 2.2ghz Core i7 might be slower than a 2.6ghx i7, but you can't compare that against a 2.2 ghz AMD Phenom as they are different architectures. Although, the core I7 will totally blow your old P4 away, as not only is it multi core which greatly benefits you when multi tasking, any apps which take advantage of the multi cores will run much, much faster, and even those that don't and only run on a single core will probably run much faster due to the increased efficiency of the CPU over your old P4.
Also memory bandwidth etc.. will be much higher, as the Core i7's use DDR3.
If you got something like that, and got a nice big monitor to go with it you wouldn't have any complaints!
Just check your software is compatible with 64-bit Vista - if you do want to downgrade to XP make sure you make a set of recovery discs first, but also be aware you'll only get use of just over 3GB of ram due to limitations of 32-Bit XP, so ther other 3GB will just be sitting there unused. Personally I'm using 64-Bit Vista Ultimate and haven't yet found an App or Game that won't run on it.