I run an AMD Athlon64 X2 processor and have been quite happy with it. In fact, I'm very happy with its performance. Like others have said before, go with AMD if you plan on making a gaming rig. Although it doesn't hurt to stick with AMD as well for other needs. At this time, it doesn't seem current Intel (Prescott) processors would be a wise choice to get in the long run due to its immense power consumption. So it may not be a good idea to go with Intel at all for the time being.
Then there's Yonah, which Injecter mentioned. AnandTech has early numbers from Yonah and it looks to be very, VERY good. However, the target market Yonah aims for is portable/laptop. So it doesn't look like we'll be seeing a desktop counterpart for a bit. When the desktop version is release, we'll see whether if it's any improvement over current Pentiums right now...
Depending on your upgrade time, it might be a good idea to wait out a bit... as the new AMD socket design is slated to be released this year. The new socket and CPUs will include support for DDR2 memory so now may not be a good time to get the upgrade bug. However, if now is your upgrade time, then by all means go for the X2 processor.
On the other hand, if you upgrade often and feel you'll eventually get a new system later on the year or the next year, hold off on the X2 and get a single-core AMD processor. You'll save some money and in the end will have enough to possibly get a dual-core processor when the new socket is more generally around.
I hope you got a good upgrade plan because all of these can factor in as to how you upgrade. Review all the possibilities and decide from there. Good luck!
