The best would be finding a stick that will work with your current stick. Regardless of where you buy it, or what kind you buy, you had better test it with your current stick before loading windows.
Anytime you mix ram you should test it first.
Here's a good program to use, and it's free: Mem test 86
You can download it from www.memtest86.com
and they have a version for floppies and cd's.
As for what kind, you might check www.pricewatch.com
for a good price, but do it soon! Prices could be going up on DDR1 ram.