Basically what I for one am saying is that you don't really need GTX's in SLI unless you have at least a 24" monitor, not with the 99% of the games on the market today. The only exceptions I know of would be Call of Juarez, PT Boats, and Crysis, all of which I'd set the cut-off point at a 20" (1680x1050) monitor. And there are simply NO situations in which the price of another GTX would be remotely justified (in a price/performance analysis) at any less than 24" with 4xAA (or higher), and even then, the games that would actually make that purchase look like a 'good idea' would be only a scant few.
Now, if you're pretty rich, then I'd say GO FOR IT. Why not? But if you aren't? Naw. Spend the dough on a 24" monitor, your 1xGTX will still work great on it for 99% of games, and then *maybe* buy another on ebay in a year or so when the games 'catch up' and you can get another one for like $120 or something.
We're also suggesting (albeit in a roundabout way) that you obsessively pour over gpu benchmarks posted on the web (like most of us geeks around here do) before you plan your purchasing moves in the future