Re: Sometimes the questions are complicated & the answers are simple but now it's rev
Depending on the motherboard, 1Gb + 1Gb might actually provide slightly better performance than a single 2Gb stick of RAM - that is, if the motherboard supports dual-channel memory and as bassfisher6522 noted, the sticks are matched. But I emphasize "slightly" because the performance gains the marketing hype suggested dual-channel would provide have not been realized in most real-world scenarios.
That said, 2Gb (regardless how it is achieved) is not very much RAM with a modern OS (plus the necessary security programs). And this is especially true if your computer is using integrated graphics. So if you are considering adding more RAM to your existing computer, I would suggest adding more than just 1Gb. If you post your computer or motherboard specs, we can help determine what your system will support. Note it is not uncommon to toss existing RAM and add all new RAM to increase total RAM. Also note that adding RAM (when starting with a small amount) typically provides the most performance bang for your money when it comes to upgrading hardware. Plus, it is the easiest hardware upgrade task for users, and rarely ever requires a larger power supply to support it.