However, I've heard of some people saying that integrated RAM isn't fully utilized in a multi-card setup.
With SLI, the RAM is mirrored -- both cards need the same data in order to push the graphics harder. So if you have 2 4GB cards in SLI mode, you have 4GB of RAM, not 8GB of RAM. But it is still "fully utilized".

For multiboxing, SLI > Crossfire, so good on you for not going ATI/AMD here. SLI works in windowed mode, Crossfire does not.

I don't have specific suggestions on cards, but the above information is helpful/important to this discussion anyway.

Other than that, generally I recommend people stick to one card because they may not be prepared for caveats that come with multiple GPUs. For example, if you decide to put on multiple monitors you may lose SLI just from that, and splitting up the two cards has performance implications as far as the different possible Window Layouts (described under http://isboxer.com/wiki/GPU_Management). One powerful card will generally give better overall options for multiple monitors because of that.