Memory upgrade

Hi all,
I am finally doing a interim upgrade (tah will last a while) with an AMD 3700 Sandy, a 7900GT 512, and more memory.
My question is would I see much performance differance going with 2 addition 512 sticks over getting 2 1GB sticks?
If the 2 1GB sticks are better then how important are the timings on the sticks?
I am finally doing a interim upgrade (tah will last a while) with an AMD 3700 Sandy, a 7900GT 512, and more memory.
My question is would I see much performance differance going with 2 addition 512 sticks over getting 2 1GB sticks?
If the 2 1GB sticks are better then how important are the timings on the sticks?