Steve, here is how it works. Both cards have the EXACT same memory states when SLI is working. This means either you'll have 320 megabytes of video memory available, from the point of view of directX, or it won't work at all.
Anyhow, both cards have the same data in memory, and each card is instructed to spend it's time drawing some of the frames. Therefore, while SLI can be much faster, you'll never have more available video memory for games than the memory of one card. It IS using the ram on both cards, but games can effectively access only one.
Barely, and even if you chose an 8800GTS SLI pair, you would need a better powersupply and would be without an upgrade path.2 x 8800gts 640mb in sli > 1 x 8800gtx, u should of went with the 640mb gts which is greater and will be gaurenteed to shine more then the 320gts in directx 10, since the 640 obvioulsy has more ram, it will obvioulsy produce better fps in directx10 gaems which have more detail and would have more ram to store info in..
Or, you could wait until 4-23-07 and watch all the fireworks when the two knock heads again. (AMD/Nvidia) I still think about my 7800GTXes which cost $589ea a little over a year ago. Now they are worth SHiiittt.
Steve, there isn't a way to make better use of the memory. Trying to do so would defeat the whole purpose of SLI. See, each card has a GPU and memory controller and ram all specced to work well together. So, one GPU running needs ALL of the bandwidth of the memory it is soldered to to work well.
So there's no way for 2 cards to each "share" a common pool of memory and hence give you the ram of both cards combined. Sorry.