What is your budget?
Basically, AMD owns all the gaming benchmarks, so unless you absolutely have to have Intel, AMD is the way to go.
Single core chips are great for gaming, and typically outperform the equivalently priced dual core chips (dual core costs more). However, this is likely to change. nVidia and others are starting to optomize applications/drivers to take advantage of the second core, so dual-core may become the gamers choice soon.
The A64-4000+ (single core) is a fantastic chip, and it costs the same as the X2-3800+ (dual core). It runs at 2.4ghz, and offers great performance (the A64-3800+ single core runs at the same speed, but has 1/2 the cache. It costs less as well).
If you are overclocking, the situation changes quite a bit. For AMD the 939 opterons seem to be the dream chips right now. The single and dual core P4s (P-D) can overclock -BUT- they require massive cooling to do so (to prevent thermal throttling).