4870X2. Or, if you can spend an extra hundred, get 2 of the lower clocked GTX 280s and SLI the fools. It will be better than 1 4870X2.
Or, if you're crazy like me, get 3 OC gtx 280s.
2 GTX 280's is only marginally better than 1 4870x2, and in some cases, the x2 actually beats the SLI setup. If you're crazy, get a pair of 4870x2's.
However, to answer the OP's actual question, the 4870x2 is the way to go. I don't think there are 1GB versions yet though - only the 2GB.