Only P4s have hyper-threading IIRC. The advantage of a dual core AMD over a single core would be that it would run simulaneous programs faster. Additionally, it would run any single program faster if that program was software optimized for dual threading (and precious few programs are, atm).
As far as P4s go, the advantage of a dual core Intel over a single, hyper-threaded P4 is that two programs running simultaneously on the hyper-threaded core would each run at half the processor speed, say, 1.8 ghz on a 3.6ghz P4. Whereas a dual core would run each program at the full processor speed, which will be 3.0 ghz on the first Intel dual cores (two cores each running at 3.0 ghz).
Bottom line: As far as gaming goes, dual cores currently hold little advantage over single cores. In fact, I would assume a single core P4 running at 3.2 - 3.8 ghz would perform faster than a dual core running at 3 ghz with the current crop of games.
I'm sure that will change in the future when games and other apps are dual-thread optimized but by the time that happens dual cores will be cheaper and more powerful--and that's when I'll buy one.