In order to redirect port 80 requests, you must use WCCP (Web Cache Control Protocol). Most cache servers support this Cisco proprietary protocol. To enable on your router, use the following commands:
router#ip wccp version 1
router#ip wccp web-cache
router#conf t
router(config)#int (whatever interface your cache is connected to)
router(if-config)#ip wccp web-cache redirect out
That should do it.