Yes, yes, today's iMac will play 90% of "graphicsy" games out there... but next year, it'll play 60% of the new games and the year following, 30%. The exact same thing was true of my 2002 iMac.
The GPU Apple puts in there is just good enough for Apple to say, "Hey, look! We can play 90% of the games out there!" An 8800GT (which Takao reminds us is quite cheap these days) will play 99% of graphicsy games this year, 95% next year and 80% the year after that.
For people like me who like to keep their computers for 6 years, I can handle a $200 upgrade once half-way through. It's much cheaper than throwing away the entire computer, which is what one would have to do with an iMac.
Anyone care to take a guess at how many games a 6-year-old iMac can run?
-Clive
First point, PC game developers will have taken note of disappointing sales of "Crysis" - only about 1 million worldwide compared to over 10 million for decent console titles, not because it was an average game, but because most PCs couldn't run it. So don't expect profit-conscious games developers to be pushing average computer specs without a significant gaming market to go for. That's why the eagerly anticipated "Empire: Total War" is expected to run happily on older DirectX 9 technology.
Secondly, if you wanted to maintain a more than decent gaming rig, after factoring in the price of just a cutting-edge video card & a copy of Vista Premium (anything less really wouldn't do in the context of your argument), you've already added
hundreds of dollars expense to your otherwise ageing PC.
Thirdly, I'll bet that 6 year old iMac would fetch more money in the 2nd hand market than your 6 year old PC would, despite the few hundred dollars of upgrades the PC may have seen over that period.
BTW, I expect my HD 2600 PRO iMac (with Apple yet to further improve the drivers) to run
well over 60% of (new*) games out there well after next year. Seriously! Later on (circa 2011 or so), like I said before, I'll buy a new Mac, maybe selling the current one for a significant return.
Once you do all the above sums (then add OS X, etc.), there are still many compelling reasons to continue buying Mac. When Windows 7 arrives (or whatever it'll be called) & if it's a much better accomplishment than Vista, I may feel otherwise; in which case (whilst keeping my Mac laptop) I'd buy a desktop PC without too many qualms... well, maybe one or two.

But seeing will be believing re Windows 7. For (me) now & the next few years at least, Mac, + Bootcamping it when I need, is the only way to go.