Well for casual gaming the new crop of iMacs with the Rage128Pro are great, the only thing that you could really whine about is UT in 32 bits where yan can expect to see lowest fps ard 10/12 in very busy maps. But it is OK in 16 bits. PC are faster for gaming and more games are available, but almost all the major ones are on the mac now and future releases should be no worse than a couple a month delays.
Personnaly I wouldn't trade my iMac SE 500 for a PC, maybe only for a cube with Radeon when they will be around $1,500 (where they should have always been).
You can't beat the overall ease of use of the mac though.
Godd luck.