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).