Well, I'd probably say that the 4870 was a better use of money, if they're the same price, go 4870, if you can save money, then maybe the 8800GTX, but I know I had nothing but problems with SLI, and well, that one or two times that SLI doesn't work in a game always seem to pop up for me, and I'm sure you'll end up seeing it. I know in SOME games it actually ran SLOWER of all things...
If I had to pick of all the cards out there, I'd say the 4870 or the 216, both cards are great value, the new 216 actually comes with mirror's edge (if you get the 231$ one off of newegg).
If your budget allows for it, I'd get that. Free game, easily worth the extra 30$, and you get physX support for the few current and undoubtedly many future titles that will support it. But it really is a toss up, pick your side. I'd recommend against getting another 8800GTX, though. Sell your current one if you can here in the classifieds, heck, the guy with the post right beneath yours wants a 75$ budget card. Maybe you can sell it to him.
I know I'd take an 8800GTX for 75$, but currently, I wouldn't recommend SLI... but if it was really cheap I'd do it for kicks.
What kind of prices are available to you currently for the cards you're considering?