I have a dual 1.8 G5 with a gig of RAM and some things run all right... but it generally isn't worth it. Games don't run, with the exception of really, really simple stuff (minesweeper, anyone?) Stay away from things with fast moving graphics or 3d, unless the graphics are only periodic and totally superfluous. Solitaire, okay. Party Poker, okay. Half-Life, no way. Definitely not HL2.
Graphics cards are not supported, except a very choice few (that are nigh useless with respect to todays games). Don't bother with games.