Performance of Multiple RAM Sticks Compared to Single Sticks

Even if you're not concerned about future upgrades, get the 2 x 1GB sticks. If nothing else, it's fewer points of failure. The speed difference between 4 x 512 and 2 x 1024 is negligible if they're running at the same clock speed and voltage.

It's an academic question anyway, since you won't notice the difference as an end-user. The bottleneck on your system is still the hard drive, whether mechanical or SSD.

EDIT:

I've been told that fewer sticks will be quicker, because of the density of the chips on each stick means less travel on the bus than with more sticks. Apparently the timings on 512s are slower than 1GBs too.