I would honestly suggest getting a PC for any games you might want to play.
I've had an iMac for about a month now...and I find it such a pain in the *** to either have to restart and go into Bootcamp for games, or use crossover, which causes weird glitches and problems in games sometimes.
Unless if you play other games like WoW, Halo, Madden, etc Which have Mac Versions. Just go get a nice cheap PC, get a decent Video card, and your good to go.
Plus the G4 Cube is really old, it wouldn't be able to Run ANYTHING that doesn't have a mac Version because of the lack of an Intel chip, and the video card being really old.