These days, for anyone that's building an upper-end system, Ifind it hard to justify not buying 2GB upfront. Reasons for this:
1) Latency differences on 2x512 vs 2x1024 is low enough now that there's very little real-word impact
2) Cost difference between 1 GB and 2 GB is minimal
3) While 2 GB might not get fully used by someone now, chances are they'll eventually get to the point where they'll want 2 GB
4) the performance impact that 4x512 has compared to 2x1024. If you want performance, getting 1GB now, and wnating to upgrade to 2GB later means that you're going to have to throw away the 512 sticks you got before
5) With AMD moving to DDR2 now as well, we should see DDR RAM start taking the major price increases that everyone has been predicting over the last year, meaning that trying to upgrade later will be much more expensive (RAMBUS anyone?).
Thus, you should probably jsut go straight to 2GB if you're debating it.
I have an MSI Neo2 Platinum w/AMD64 3200+ and GeForce 6600 GT, Antec TruePower 550 running 4x512 Crucial Ballistix at 2-2-2-7 1T at a "mild" 2.15GHz overclock. I'm quite happy as it runs HL2 superbly. My point is that Crucial Ballistix single sided DDR2 in whichever density (size) you choose offers you the choice of a moderate, or 2x moderate memory purchase. Also, certain progs (Adobe Photoshop, Illustrator) suck all the memory available, so 2GB is better than 1GB.