I guess your Powermac isn't cutting it game wise eh? Well, first you want an SSE3 compatible prossesor (so when (if) OSX comes for X86 (openly), you can use it). So you have intel or AMD, for overclocking and better performance, not to mention cheaper, go AMD. You want a 939 pin vience core CPU at least 3200, if not 3500 (3800 if your wallet allows). For a motherboard, get a gigabyte,DFI,ASUS,MSI, and maybe a chaintech. Make sure it is a 2000mb/s board. For ram, pretty much anything will do, you will want least a gig if not more, and maybe in two 512 sticks if you want or one 1024 stick, i don't care. No reason to buy anything faster than 184 pin DDR400 because amd won't recognise it as any faster. Then for hardrives, at least a 120gb 7200rpm SATA drive will work, bigger the better there. Then you will need a video card, so make sure you have pci ex on your motherboard, you can do agp, but you have enough $$$ for PCI EX. Wallet permitting you may want SLI, especially if you do some real high resolutions. You want at least a 6600gt for this, 6800gt or 7800gt are better. Now i assume you already have some 5.1 speakers, if not, get some. Itegrated sound sucks, so get a nice Creative labs sound card, probably the new series (forgotten name). and get and old case and a good antec power supply. That my speial. PM if you have questions.
jeff