Anandtech compare all three solutions here.
I'd go with the HD 4870 X2 because:
- It's basically two HD 4870 1GB's in CF. A single HD 4870 512MB is slightly faster than a single GTX 260, so two HD 4870 1GB's will only widen the gap.
- It's a single card, and it doesn't take up two PCI-E slots. You don't need a god awful Nvidia motherboard to run it and it doesn't take up as much space.
- It's the most awesome card ever.
EDIT: Anandtech used Catalyst 8.8 in that review. 8.9 is now out and offers a few performance improvements especially in a CF configuration (like the X2).