go with GX2 if you're not planning to do Xfire, this card is definately out perform a X2 + 3870. Test results shown from Anandtech reviews. The cheapest X2 is around $440 and a 3870 is about $200 (depends on which versions) = $640 but you can get GX2 for $600 and the driver will getting better and for sure they will go on sale or rebate later on. You still come out $40 + performance ahead with this card.
Quad X2 is not worth it.
The question is; How well the upcoming HD4000 X2 do against this card? Do you a patient to wait to find out???
Here's a quick summary for most @1900x1200:
Game Resolution and Settings 9800GX2 vs.3870x2+3870
COD4 1900x1200 4xAA,16xAF 84.2 vs. 79.9
Crysis 1900x1200 4xAA, 16xAF 34.7 vs 31.9
UT3 2560x1600 0xAA, 0xAF 80.6 vs. 90.2
Quake Wars 1900x1200 4xAA,16xAF 101.9 vs. unsupported
HL2 Ep2 1900x1200 4xAA,16xAF 96.7 vs. 87.5
Power consumption (W)
Idle: 185 vs. 179
Load: 351 vs. 453