2 X 512 will RUN faster than a mismatched pair of 512 X 1Gb. If your motherboard supports dual channel it requires a matched pair to run in dual channel mode. The mismatched pair will run but not in dual channel mode. Because the mismatched pair is more the cpu will still read first one stick and then the other, (instead of both/Dual at the same time) but it will be a little faster because more can be stored on it closer and the cpu doesn't have to go to the hdd as often and have to travel as far and wait for the hdd to find it and relay it back. If you can get 2 sticks of 1 GB each as a matched pair (as everybody suggests) you will be better off. But if your only choice is to get the 1 stick of 512MB or 1 stick of 1gb?? - then go with the 1GB stick.
If you're interested HERE'S A LINK to NEW 1GB PC3200 400Mhz DDR NON ECC DESKTOP RAM MEMORY for $11.57 each. Maybe at that price you can get 2?
GeekApproved - you scared me, The first 3 times I read "In most cases 2 x 1gb would be best, but if your running Vista or Win7, then 1.5gb would probably be better" I thought you were telling him 1.5 is better than 2.0 for vista and 7. Finally figured it out though