For five boxing on a single computer, I really would recommend using a Q6600. The "Extreme" Dual cores are quite over-rated. Lets look at it objectively, what are you getting? For one, TWO LESS CORES, an increase of 266Mhz, a faster FSB which is admitedly useful but only really if you are using fast RAM which you are not.

The Q6600 is a dream as far as I'm concerned, it overclocks beautifully even with a stock cooler/fan combo. There are very very few reasons to get an E6750 over a Q6600 other than the minor diffrence in price.