The short answer: Yes.
The Long answer: Probably.
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.