The Phenom line of CPUs has the potential for great performance. Unfortunately, problems with getting the chips to run at a high enough clock speed means they can't compete with Intel at the top end. So, if you're planning on a full system upgrade, TrustedReviews would recommend going with an Intel chip.
If you've already got an AM2 motherboard and some decent DDR2 RAM and you just want a CPU upgrade then dropping £150 for a Phenom 9600 Black Edition will get you a nice boost in performance. However, TrustedReviews would probably still recommend you wait for the release of the next line of Phenoms, which should clock higher.