If you had the choice between 2x 512MB and 1024MBs and the motherboard does not support dual channel then go with the 1GB module. There isn't a difference in speed between the two. The motherboard only sees the memory as a whole not as individual parts.
If the motherboard supports dual channel then buy the 2x 512MB modules. You will get about 10% increase in performance.
Crucial Performance Lab
The Memory Experts(SM) at Crucial Technology is now on board to give you straight answers to your memory-related questions.
When responding please provide your full system specs; hardware and software, OS, BIOS, drivers Etc. You can e-mailmemory related questions to firstname.lastname@example.org