With 2 under SLI, you'll easily be able to max out all Direct X 9 games out today, I'd imagine.
You may start having problems with Direct X 10, but at least games then will be made to function with SLI from the start, so with your current setup, you may have an advantage over one card.
Its only 320mb shared, BUT, like people have said, if you are willing to sacrifice resolutions, and/or texture resolution/quality, they'll go a long way.