On many motherboards you will get a noticeable performance hit, especially when overclocking the memory when you use 4X512 sticks. For example, on my Asus P5AD2-E Premium system, I lost over 50 Mhz overclocking when I went to 4 sticks. So I got 2X1024 and all is well again. If you need 2 gigs RAM, make sure the RAM performs equally well at 2 Gigs also, so stay with OCZ, Corsair, etc. RAM and you should be fine. If you don't overclock with the tightest timings you can achieve, then by all means use 4X512. There are very few apps and games that require 2 Gigs anyway, but that will all change with the arrival of VISTA.