Yeah, I know. Thats whats holding me back.
Sorry for just reformulating your original post and not answering your question at all.
I like to buy and sell a lot, I don't know if you're the same but if you're good at it, and you do it at just the right time, you can upgrade for pretty much $0 if you are OK with using eBay.
For example, when the HD 4850 came out, they were the same price as the 8800GT, I quickly put my 8800GT on eBay and bought a HD 4850 from there as well. I drew even and spent nothing yet I got an upgrade. If I were you, I wouldn't go SLi. But rather than spending another $100 on an 8800GT, I'd cash in on the one you've got when the HD 4770 comes out (They should be selling for the same price) and then buy a 4770 as soon as possible. There, you've upgraded for nothing. It's not going to be as big as 8800GT SLi, but at least you'll still be free to choose whichever chipset you want.
You may not want to take risks though...