If it comes with 2 matched sticks of 512MB and you want to go to 2GB I'd suggest just getting another two
identical sticks of 512MB rather than binning them and getting two new 1GB sticks. Unless you're running a webserver there's not much point in going over 2GB even with Vista
Even Linux, which uses memory more intensively than Windows (for speed and efficiency) won't ordinarily benefit much from > 2GB
But be careful, fast dual-channel setups are quite sensitive to minor variations in memory timings so be sure that the sticks are matched.
Many, many systems are unstable not because of the OS or duff drivers but because memory modules are not properly matched or timings are set too aggressively in the BIOS. Your system
must be able to run Memtest86 -
http://www.memtest86.com/ - overnight without errors or it will never be stable.