Clockspeed (amount of ghz) hasn't been used to determine a processors speed/efficiency for a long time. Most notably since the mid 2000's In those days AMD Athlon was far superior to the Intel Pentium 4, yet intel processors ran at a much higher clockspeed than the AMD models. Determining which processor is actually the best comes down to so many factors, from cache, to actual features of the processor, which can increase the speed, and efficency of them quite significantly. This is why a 4core intel i7 running at 3.0ghz, is generally considered better than a 8core AMD bulldozer running at 4ghz. It's a difficult question to answer simply, because of the complexity of the actual processors. It's a bit like using graphics memory to decide which is better between two very differnet graphics cards, it should only be used for 2 comparable processors. The easiest way to determine which processor is best, is rather than relying on numbers, and specifications, actually look at benchmarking results, and then go with which scored the most. These are real world tests, using real peoples computers.
As for number of processor cores, it's all very dependant on the actual processor core, its featureset, architecture, how much cache each core has, and even down to the bus, and the motherboard northbridge, and its featuresets. Like just imagine, for theories sake an 8 core Intel Atom processor (these don't exist obviously) But it wouldn't be anywhere near a dual core i3 processor, even though it has a lot more processor cores. By the "the more cores and higher clockspeed is better" theory. a 3.0 ghz Core 2 Quad, is identical to a 3.0ghz i7. That's far from the case.
Which brand you choose comes down to this. AMD is usually best prices vs performance. Intel are usually just more powerful, and for a gaming system despite what the other poster said Intel offers the best performance. Yet in other scenarios, AMD is going to come out ahead.
EDIT: Also another point, that's just hit me. The vast majority of games don't really fully utilize a quad core processor to its full potential, in most cases you'll see absolutely no performance increase from a dual core processor, to a quad core processor. So when you start talking 8 cores, that's most of the processor, sat completely idle doing nothing. So for gaming each individual core matters far more, than the combined processor. Most games are far more graphics intensive, than processor intensive, generally as long as you have a decent modern processor (anything better than a pentium D) You'll be fine, it's the graphics card that really matters.