|Just look at the benchmarks here, there are many more around aswell. There was also a great one on Toms Hardware, but I can't find it ATM. |
As you can see, most games do not support SMP, or barely support 2 CPU's in SMP. For the money, unless you are encoding videos, or playing in photoshop, you will not see any increase, and will most likely notice a slight decrease in performance when going from a dual core to a quad core. Especially if the dual core is 600Mhz faster, it will leave the quad core in the dust.
It's all in the benchmarks.
EDIT: Also take note on just how little CPU performance matters in games, for example, even a 700 mhz difference only showed .3fps on one of the benchmarks. It's ALL in the video card, spend your money there!
PowerMac 9600(1 ghz G4)
ATi 9200 PCI