Wooley, it's a rule of thumb based upon the fact that all else being equal, the memory used on the 512 cards
generally is lower wait state that that used on the 1 GB cards (just like when buying system memory, you will find that most 4 GB modules have more wait states than a comparable 1 GB, etc). Obviously, it's not always true, or even necessarily must be true - but generally, it is.
Note for example that these two EVGA cards appear identical on NewEgg other than 1GB vs 512, while the specifications from the web site tell a subtly different story - the memory on the 1 GB card is 1.0 ns memory and that on the 512 is 0.8 ns.
http://www.evga.com/products/pdf/512-P3-N973.pdfhttp://www.evga.com/products/pdf/01G-P3-N981.pdfSo unless you are playing a game that either optimizes differently for a 1GB card vs a 512 MB, or that requires more than 512 MB, the 512 card is going to be faster. And of course if you add in a 32 bit OS which can give you a double whammy because the extra memory reduces overall usable memory, the 1GB can degrade relative performance even further.
Now, it's probably obvious, but if you were playing a game that required more than 512 or could even make use of more than 512 MB, the 1GB card could easily outperform. But most games these days do not (yet, at least - or at least that I'm aware of).
<S>