And those are just a few of the threads I happened to have replied to yesterday.
In essence: paired RAM is a bit faster for a given amount of RAM, but more RAM is almost always faster than less RAM. 1.25 GB will be way faster than 512MB matched.
First suggestion is you tell us what machine you have, it makes a difference.
Second suggestion - use the Search function of the forum to look for recent articles covering the same question you are asking.
On the intel Macs, the machine will utilize dual channel mode of the memory buss when you have matching chips. This gives a small but real speed increase. However, the performance hit you suffer from not having enough RAM is much greater, so more total RAM trumps matched RAM.
1 Gb is the minimum for decent performance, if you are using professional apps under Rosetta, you will want 1.5 to 2 Gb RAM.
So get the 1 Gb module now, and upgrade it to 2 Gb when finances allow.