I don't know if I can use both GPUs (9800 & 660Ti) on my mobo with SLI
for SLI you need strictly need to use 2 exact card. for example if you want to SLI GTX660 Ti the second card must be GTX660Ti as well. but you can combine the two making the GTX660Ti as primary card (rendering) and 9800GTX+ as dedicated PhysX card.
can we use 2 GPUs on one board ONE is PCIE 3.0 & the other one is PCIE 2.0??
yes. for such purpose that i mention above or
just add a second card on the extra PCI-E slot to add monitor output