When asking about such things it's a good idea to give details such as model numbers etc. The "(I think)" doesn't exactly help narrow things down.
Though PCI-E is backwards compatible (excluding some rare cases of v2.1) so there's little chance of having any problems there, unless it's a crap motherboard which I don't know from those details in which case you might have a power shortage problem if the card doesn't take power directly from the PSU.
+1, but sometimes with a significant performance drop.
I had a 9600 GT in a PCI-E 16x 1.1 slot, and it worked well, but when
I upgraded to a Mobo with a PCI-E 16x 2.0 slot, it's 3DMark 06 score
almost doubled.