3870X2 in Crossfire better than?

Is 3870X2 in crossfire better then one 4870X2?

Last poster is absolutely right. 4870X2 will outrun 3870X2 CF anytime any day. The more cards you add to crossfire, the more CPU overhead there is and the less each added card scales. I actually had quad 3870 CF, and I ran my tests with Athlon 64 X2 @ 3.24GHz, in Crysis, running on High 1920*1200 in Vista 64-bit, I had results: Single 3870 17.88fps, single 3870X2 21.50fps, 3870X2 + 3870 GDDR4 21.24fps, Quad CF 20.69fps. This is all on built in benchmark Harbour, which simulates actual gameplay pretty well. Clearly my processor speed was limiting my Quad Crossfire severely.
Another thing to keep in mind is that Quad Crossfire, or any CF, doesn't scale in new games until latest drivers come out. Again in my testing, Far Cry 2, all details set to ultra 1920*1200, results were (in same order): 29.11fps, 26.63fps, 20.65fps, 15.73fps. This was on Ranch Small test, run using built in benchmark. As you can see, there is a very steady decline in CF in this new title, and I was using a hotfix 8.10 driver from ATI. Doesn't look like it fixed much, since a single card beat out all other configurations.
On top of this, I have currently experienced some glitches in DVD playback when it slows the video feed, as if it's running 5fps, clearly a CF problem. THis only happens on Quad CF, not dual, so there. Choose the Dual 4870.
Ohh and if you're getting 4870X2, you'll also be getting a larger frame buffer of 1GB per chip. Better than 3870X2 512MB per chip.
Hope this helps.