To be honest - it really depends on the game, so there isn't a set % boost that a quad core will give over a dual core. It also depends on the architecture, as a stock i7 930 will still thrash an Athlon II X2- even in a single threaded CPU test.
Most games are fine with a reasonable dual core, however there are an increasing number of games these days that really do make use of three or four CPU cores (BF:BC2, Civ 5, ARMA 2, DA:O, GTA to name a few). In these games you can get a considerable performance boost over a dual core CPU, so long as your graphics card is up to the job.