SLI bottleneck?

My motherboard, ASUS M2NSLI-Deluxe supports 16x PCI express for the primary PCIx slot, and the secondary is only 8x. Currently I have a gtx260 in the primary slot.

This rig is on it's last season (Socket AM2). By Christmas (when prices drop) I will be on an i7 platform of some sort but in the meantime would adding a second gtx260 be a waste?

-I play at 1920x1200.
-Crysis, l4d, cod4, fc2, css ... pretty much any/all worth while first person shooters.
-Crysis is barely averaging 40fps at full resolution with full settings (no anti-ailasing or filtering)
-AMD X2 6000 (3.1ghz)
-4gb fast corsair ram (dont remember)
-Vista64 ultimate
-Power supply is capable, forget which one i put in there
-antec 900 case (dropped my gpu temps by a solid 15-20 degrees when i replaced the dinosour case i had)

All I ask you folks to avoid is how i need to go ahead and upgrade with the i7. I know this.
(And im not going to tell you folks not to preach ATI, cuz day by day they're positioning themselves to kickass)

THANKS