Isn't that a pretty ignorant question for someone with 11,000 posts? This kind of thing gets asked almost every day, and you should know that you won't get any good answers unless you tell us:
1) how much RAM you already have (and what kind, and what configuration)
2) what motherboard you have
3) if you are overclocking (and what speed, and if the memory is synchronous)
4) if you think it is likely that you'll need to upgrade beyond 1GB again before you upgrade the whole system (at which time you may move to DDR2)
5) since you said "another system," this implies it isn't your main system and is probably just a gaming machine that doesn't see much multitasking, so can you really see the machine benefitting from over 1GB in its usable life?
But then, if you thought about those things, you probably wouldn't have asked the question to begin with.