Kinda depends on the RMS on the speakers, but something like:
http://cgi.ebay.com/NEW-ALPINE-MRP-F450-4-CHANNEL-AMPLIFIER-CAR-STEREO-AMP_W0QQitemZ350072586608QQihZ022QQcategoryZ39739QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
would probably work fine for you.