what should I be looking for in a processor, speed or cores?
to lessen up your choices just look for an intel.
a gamer, dual cores will do, higher clockspeed is what you should consider.
if you do more than an average user do like animations, video editing and image rendering, more cores the better.
If you can afford to spend a little more money $100 - $200 at most, then go with Intel.
Intel's CPUs are currently the best when it comes to overall performance. Clock for clock, they are more efficient/powerful than what AMD has which is why even a 4 core Phenom II X4 955 BE costs less than 2 core Intel i3-2100 (the slowest Sandy Bridge i3 CPU).
Will AMD's Bulldoze core family of Zambezi CPUs change that? It remains to be seen...
It depends a lot on what you're doing, my Phenom II x6 does good enough for me in games, but only truely shines in heavily threaded work, such as video compression. If you only play games a 6 core is a waste, you're better off looking at higher clocks, or more effecient arcitectures (such as core Ix)