I assume you would pay off the balance first.
The immediate impact would be from losing the card's CL in your total CL, so your util might creep up.
As for history, the card *should* keep reporting for 10 years after closing. If your next-oldest card was opened soon after the Cap 1, and you plan to keep it open, you might have little impact in 10 years when the Cap 1 falls off.
fused has a good thread on this, which might help you think your way through:
Closing Credit Cards
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007