2 questions for yourself:
- Do you care about DX10.1?
- Do you care to spend a LOT of money for the bigger card?
I'd say go for the 2nd 8800GT: Cheaper than ever and still giving good performance.
Now, i don't know much about nVidias cards, so i don't know if your PSU would hold up with 2 8800GTs. If it can't, then see if going for a GTX260 costs more than the 8800GT + what-ever-watts-you-need PSU. (added)
On a side note: I have a friend with a rig close to yours (already 2x8800GTs and 900W PSU) and i can tell you it's a fine machine.
EDIT: Added idea that came up.