It says the min videocard required is ATI Radeon X1800 what would happen if I tried it using ATI Radeon X1600 would it just be laggy / slow loading or would it not work at all.

I want to try it out before I go and buy a new video card.

It would probably run the game but performance may be a little lower, and minimum specs are usually a guide line to aim a little higher.

You might have to turn settings down, but a Radeon X1600 seems like it should be just fine for this game.