It will work. The only difference is when you need extra brandwith like when recording a game with fraps, you wont be able to do it. Except that it will run fine.
That ^ is not right, the GT 630 will not saturate even a PCIe bus, it's not a very fast card. You'll be able to run it just as well on any slot. Even if you have a PCIe 2 slot FRAPS or anything else won't run any better with a GT 630.
I have a gts450 and cannot record with fraps with full screen
That has nothing to do with the PCIe slot though in his system. It's an issue with how FRAPS records. If you look around, almost everyone that uses FRAPS to record has issues with recording quality. It would be even more so for a GT 630 card that is slower to begin with, on a system that is probably slower than yours.