The Inq tests what happens when you upgrade your memory amount with Windows Vista from 512MB to 1024MB, 2048MB and 4096MB. Their conclusion seems to be that Windows Vista can't run on 512MB memory if you want to play games. They recommend 4GB of memory for optimal performance.
You simply cannot play games on Vista with 512MB o memory, at least not the games we tested. You will sometimes get playable scores but even then the experience and the load time will be ridiculous.
Anything more than 512MB will help you a lot, with 2x1GB being the best price performance buy. 4x1GB is the best choice for best raw performance. It is hard to prove it in every test, but if you have 2x1GB and you load as much in memory, the system will become endlessly slow