If you already have an 8800gs and sli motherboard, then your cheapest and best upgrade would be another 8800gs. Newegg currently has the evga 8800gs for $80AR shipped. I got mine for $65 with a coupon code, but that's expired now.
Most current games, it takes a higher resolution to make the single card struggle, but SLI can benefit any time you are GPU limited.
If you had an SLI mobo and 8800GS already I would say at 1600x1200 for sure SLI the 8800GS (I am myself). But seeing how you do not have a card now, then you have more options. Two 8800GS will pretty much beat anything under $200 in most games, so it's a good choice. But, the HD4850 looks like a great option too. A single 9800GTX (if priced about the same) is a good option too. Otherwise, if you want to stick to NV the 8800GT or GTS (512MB) may be your best bang/buck if going for a single card now.
No problem. Keep in mind scaling results will vary by game and even area in a game. (edit: example - Oblivions towns are more CPU limited and this kind of scaling would not happen. ) Not to mention system specs, resolution, eye candy settings, etc. I'm hoping to go more in depth someday and do a good dozen or more games with a few SLI combos.