Old AGP card doesn't fit in slot. Right key though
 

My motherboard (ABIT VA-20) AGP slot has a clip for the registration tab of the video card. A registration tab looks like this: third picture from the bottom
My CT5823 NVidia Riva TNT2 32MB AGP Video Card with TV-Out card has no registration tab. Look http://www.alancomputech.com/6001099.html There's a big rectanlge instead of the registration tab.

Should I cut it off?

On my AGP slot there's a clip for the registration tab, which is in the way for the rectangle. It won't fit in eventough it has both the 1.5V Key and the 3.3V key

I cut it off
 

I cut it off and it work fine so far. The screen hasn't frozen yet.

I think it was brought in after a while on AGP cards + boards to prevent the card being accidently removed (duh, who'd do that whilst it was running?), meaning you had to flip the tab on the board to release the card. If it's an older APG card, which it sounds like, might not have this.

I would not advise cutting into any board to make it fit into a slot. Personally, I agree that there seems to be little real need for a retaining clip, but even so, if a board doesn't fit into a slot, it's best not to force it or otherwise mess with it.