Running 2 cards in SLI with 1 gig IS going to show a better relative performance gain with more memory over running just 1 card with more memory. Adding another card in SLI spreads the load on the GPU's and allows you to start cranking up the resolution and eye candy, and make use of some of the extra memory.
Also, the 256 bit memory buss becomes a bottleneck for over 512 meg of memory. That is why the 8800GTX with 768 meg of memory has a 384 bit wide memory buss, and these cards are still king of the hill when you really start cranking things up.
In other words, on the single card, the extra 512 meg of memory won't do you much good, as you will simply run out of GPU power first. In fact, at 1900x1200, a 512 card actually outscores the 1gig card in many scenerios.
But don't take my word for it, here are some benchmarks to prove it:
http://en.expreview.com/2007/12/30/review8800gt-how-many-memory-do-you-need%EF%BC%9F/