I just built a new PC and had a similar choice. I agree with the 4870 1Gb or GTX 260 "maxcore" as the best contenders. You get the fleixibility of being able to add a second card later if you wish (and have chosen a suitable motherboard).
Failing that, 2 x 9800GX2's offers a good price-to-performance ratio, but they'll need a lot of power and presumably chuck out a lot of heat - there's 4 GPUs!
Best place to check is Tom's charts. They do a price-to-performance guide, updated monthly. Asking on forums is risky - there's a lot of fanboys out there! Don't ignore factors such as noise, heat and power consumption if they're likely to bother you later!
Isn't a 4870X2 essentially a single-card Crossfire solution of 2x 4870 1Gbs?
And a such, you won't get any benefit at all in non-Crossfire games? I've yet to see much to convince me of the need for a 4870X2 other than running at very high resolutions (24" monitor and upwards), in a Crossfire-supporting game, with all the AA bells and whistles turned on.
For most people a single 4870 should be more than enough (probably even a 4850, as noted above). You can always add a second card later if you feel the need (2x 4870s run as well as a 4870X2 in the benchies I've seen) when the cost is lower.
If those are your only options, the 2x 9800 GX2's would blow away SLI'd 8800's. Just make sure the PSU can handle the load (800 Watt PSU, if not 1kW minimum).
A single GX2 is a force to be reckoned with, two of them gives a lot of power. Note though, there are a few games the GX2, due to its design, performs worse than its capable of. Two of them should negatate that downside a bit, but its something to watch out for...
The cards can get a bit hot; I used rivatuner to OC my fan to 80% fixed, and i'm steady at 75C. Be sure your case is capable of getting rid of that hot air, or temps will higher than you would want.
BTW, 2x 9800GX2's go for the same or less as a single 4870X2. I would love to see benchies between those two matchups...