VIA Arena take a good hard look at AMD's dual core CPU and how it does and/or does not benefit gamers:

If you are a die hard competitive gamer and every frame per second counts and you want the extra features turned on like AA and shadows then MHz is still king. This may change as the year draws on, but everything I have seen in testing shows that the higher the MHz, the better the fps. So out of the three CPUs I have tested here, the Athlon 64 3800+ is the goer for hardcore competitive gamers.

If you like playing games but you also use your system for other applications like ripping music, video editing, graphics editing, burning CDs etc, I would definitely recommend a dual core over a single core. The performance increase in multi-tasking outweighs the drop in fps. Dual core truly comes to life if you want to play World of War Craft whilst chatting on Teamspeak and ripping a DVD all at the same time. Windows is quite capable of allocating the appropriate applications to each core to spread the workload.

I was surprised to see just how competitive the Athlon 64 3200+ was next to the Athlon 64 X2 3800+ in gaming. If you are on a budget then the 3200 is a great deal since it is over $100 cheaper than the X2 3800+. If you are concerned with future proofing your system for applications and games that will use dual core technology, save your $100 now and put it into a dual core CPU next year when prices have fallen through the floor and we will probably all find lovely whopping 4600+ X2s at today's 3800+ prices! Even if you put that $100 you saved into extra RAM, you would see a greater performance increase right now that putting that money into dual core right now.