QUOTE(Dedeleko @ Sep 13 2008, 12:46 PM) [snapback]1464696[/snapback]
I think 2GB total memory should be enough. What you are mostly using the PC for?
I would say go for the single 1GB stick DDR2 PC2 - 4200 ($20).
I would not recommend the DDR2 PC2 - 6400 because it would not be worth the extra money. Your current memory is DDR2 PC2 - 5200. When you mix different types of memory sticks they will operate at the speed of the slowest stick (In your case 5200).
If you buy the 4200 stick it will also slow it down, but you will not notice a drop in performance. In fact you will notice an increase in performance. Memory capacity has a much bigger performance impact than the speed. Especially when the increase/decrease in speed is very close to the next.