What are your system specs, and do you only have 1 chip at the moment?
Depending on the system, 2 chips is ideal, more helps but not as much as 2, particularly with dual chanel Athlon's (939 pin).
Assuming you're not running something anchient, you'll probably get a good full system performance bonus out of going to 1 GB. Going to 2 GB won't give you as much of a performance boost unless you regularly stress memory (Either with particular games, rendering, data processing, or such applications).
1 GB is probably a good sweet spot now for gaming. 2 GB will help a little more, more in some games than others, but you could be talking the price difference between 1 512 MB chip to 2 1 GB chips, so the returns may not be worth it.
Personaly, I've been running 1 GB for over 2 years now in this system and a previous system. I notice it boots a bit faster, I get better HD performance because the system doesn't go to the page file as often (And I save performance there by putting a static 4 GB page file on the HD), and even on a single core CPU multi-tasking was easier. For a dual core system, I'd lothe to think of less than 1 GB.
Good stuff.... Those sticks of memory you are going to buy is what i have, and i'm at the moment trying to OC them nicely. Bront is doing an OCing journal, which includes his Corsair memory modules, so check it out if you want to find out what sort of settings are stable with that sort of memory. Bront's OCing Journal