RAM upgrade

[tidefan94]

I currently use 3 gigs of DDR3 Ram for my i7 system. I want to upgrade my Ram as seeing that'll probably give me the biggest performance boost (and that I'm upgrading to Windows 7 64-bit this fall). What I want to know is if I should just buy the same 3 sticks I already bought and put those in too (my mobo has 6 slots) or just buy a 6 gig set (that only has 3 actual sticks).
My mobo is: a gigabyte EX58-UD4P

 

[tidefan94]

Could anyone please help me, because I have to order it tomorow.

I was thinking of buying this

http://www.newegg.com/Product/Product.aspx?Item=N82E16820220365