A single GB dimm will allow for better future upgradeability, since you can get another matching dimm later for 2 GB.
2x512 will likely perform better, since you'll have dual channel, & timings are always better on 512 MB dimms.
That being said, to get 2 GBs in dual channel later (if you plan to upgrade), you'd need 4x512 MB, which will have to run @ 2T instead of 1T.
Frankly, if you don't feel you'll need more RAM later, i'd just get 2x512 MB.
But considering how games are becoming so RAM hungry, it's only gonna be a matter of time...so i'd get 2x1024 MB if you can afford it.