What do you mean, better?
Assuming that your machine is a Core2Duo:
2 x 1 GB gives you Dual Channel access, which is 6% - 8% faster on average. IF you don't load up more than 2 Gb of programs and data.
1 x 2 Gb SODIMM gives you the opportunity to go to 3 Gb RAM -- which means that you lose out on the Dual channel 6% - 8%, but stand to gain more performance than that if you are regularly using more than 2 Gb of programs and data, or are using programs like Photoshop that can exploit 3 Gb RAM for faster caching.
There is a
Guide on this ...
and this is the 24th Intel RAM thread...