Macs have been about 20-40% industry standard parts for a while now. I've got a 5 year old mac with 768MB of PC133, two IDE CDROM drives as well as 2 IDE hard drives (granted only ATA-3 but still ATA). The power supply is a standard 250W ATX power supply and my extra NIC is a generic RealTek RTL8139C card.
More recently, Macs have been using PC2100/2700 RAM and are up to ATA100 (or 133, I can't remember). The power supplies are a variant of the ATX standard that supplies an extra 28v rail for powering Apple's goofy ADC port (DVI, power, USB in one cable, simultaneously their most brilliant and dumb idea in recently history). Apple requires very, very exact matches to the JEDEC standards for memory, so some of the things that RAM makers do to cut corners can make things hit and miss. I've never gone wrong with Crucial memory tho.
As far as games go, well, I don't usually play games on my mac as it's my WORKstation (I have a Wintendo for games). I do know that there is a small selection of decent games. Granted, there's about a 100:1 ration between the number of PC games to Mac games, but the bigger titles (Quake3, Unreal Tourny, UT2K3 [soon], etc) are out there.
The problem with Macs is that few people know PPC assembler very well. I think the best quote I've ever heard came from Graeme Devine's .plan file a year ago regarding writing an update for Q3: "My battlecry of late has been 'Somebody find me a book on PPC assembler NOT written by Motorola!!!'" The body of knowledge is just not very big and many programmers are forced to use awful/unoptimized code.
You'll see that programs that are ultra-optimized (like the RC5-72 client or similar small programs) that can make use of Apple's vector unit (Altivec or Velocity Engine) are blazing fast. As a benchmark, a P4 1.8Ghz runs a little under 3Mkeys/sec in RC5-72 while a 1Ghz PowerBook G4 (laptop) crunches a little under 10Mkeys/sec.
Of course, a platform can only be as good as it's developer support. The developer support for the Mac platform is very solid (almost rabid) but it's also very small. It suffers from being a closed system.
As far as swapping a Mac card to a PC, that's hit and miss. You definitely need to flash the bios. Mac graphics bios files are typically larger than PC graphics bios files, so it's usually easier to go from Mac to PC than vice versa (which is kind of silly to do since Mac graphics cards cost anywhere between 15% and 50% more for their identical PC counterparts). I'm not certain about the 9700Pro specifically, but any info that you need could be found at
http://www.xlr8yourmac.com and it's forum.
Apple is a BTO nazi sometimes. Their BTO options were great, once upon a time, but cost saving measures have caused them to streamline their operation, limiting BTO.
Hope this helps.
Jason