The 8800GT is a 512MB card and the 8600's would be 1GB each.  

If you're building a system, probably the one card.

Granted, the issues aren't that bad, but there can be some issues with an SLI rig.  You'll also need a more expensive power supply, and more expensive motherboard.

Look at the performance graphs.  Don't just multiply card X performance by 2, because you don't get that level of performance in reality.  You need to see a performance benchmark with X card in an SLI setup.

It's worth it if you're building an uber-rig with two of the best cards for the best possible performance.  But an 8600 is a mid-range card, and an 8800 is much more powerful.

ETA:  Don't be fooled with massive amounts of memory on a lower-end chipset.  
1 GB on a single 8600?  

The card's GPU can't handle textures that large, and will be a bottleneck anyways.  You won't get mega resolutions out of that card (not at good framerates anyway).

8800 GTX x 2 in SLI Mode

Quoted: 8800 GTX x 2 in SLI Mode

Dayum!  Now we're talkin' SLI!