Do both ISP links terminate on the same router? If so, you can load share outbound traffic by configuring two default routes, one pointing out the serial interface and one pointing out the Ethernet interface. You can then enable CEF, which will do per-destination load balancing across equal-cost routes by default. Note that this doesn't affect inbound traffic, however. Inbound load balancing is more difficult, and depends on the details of how your network is setup. Do you have different address blocks from each ISP?
Failover over between ISP's is another matter. If you have point-to-point serial connections (T1, T3, etc.) to an ISP, you can generally rely on the serial interface changing state to "down" when there's an outage. This is enough to remove the default route pointing out that interface from the routing table, which causes all outbound traffic to exit via the other ISP. With one of your ISP handoffs being Ethernet, however, things aren't quite as easy. You're probably going to need to run a routing protocol (BGP, for example) with both ISPs to detect outages. One possibility is to have each ISP advertise only a default route to you. When a link fails, the BGP session with that ISP will go down and the default route advertised by that ISP will go away. This should give you the same functionality as two static default routes pointing out T1/T3 interfaces would.
I'd suggest searching this message board, this site, and the Internet as a whole for information about ISP/BGP load balancing and failover. There's a lot of information out there on this issue (it comes up on messages boards often), and it's not a very simple topic. The book 'High Availability Networking with Cisco' by Vincent Jones is also a good resource.
