as its an older i7 platform stick to 1333 max ram speed or you will be limited to 1 stick per channel so 3 sticks max.
if you stay under 1333 you will be able to use all 6 slots, but jump to 1600 and half your ram will be ignored by the bios and windows will randomly loose portions of it...
look up balistix ram. i got 12 gigs for 52 quid 3 months ago. from ebuyer. but the price has gone up to 82,,, still its a bargain compared to most suppliers.
http://www.ebuyer.com/search?a02004=PC3+10600&a00062=12&cat=3&store=2
for my system which is also ex58 based it was a very good deal. as it was retailing elsewhere at over x2 what i payed. because im running only 3 sticks i can run it at 1440 without a voltage bump (they run at 1.5-1.6v) and i managed to tighten the latences to get that little bit extra... so like i said very good ram...
for you though you could probably run if from the standard 9-9-9-27ish timing to 8-7-7-22@1333 and may even get the cas 8 down to 7. giving an extra 2-3% performance gains which puts it very close to ddr3 1600 9-9-9-27 ram as far as system performance goes.