RAM setup matter for performance?

I currently run 256mb of pc2100 (kingston brand) and I wanted to make the jump to 512mb but I realized that maybe I should go for 1gb if I will be video editing on this PC since I just got premiere. I have 3 DIMM slots on my K7T266PRO2-RU and was wondering if 256, 256, and 512 would work just as well as a setup of 512 and 512.

I think generally speaking (with exception of dual-channel motherboards) fewer sticks of memory is better, and unmatched pairs can cause problems. Also check your manual / mobo webiste because some boards can only use more than 2 slots if memory is ECC registered or of a lower speed.