You have several options here depending on your environment and goals. I'd recommend searching the forum for 'load balancing' to see past discussions on the issue (there have been many). In a nutshell: load balancing outbound traffic is fairly easy -- the easiest way is to point a default route out each interface and use CEF to load balancing either on a per-destination or per-packet basis (the former is usually a better idea). Load balancing inbound traffic generally requires the use of BGP, which can be an art in of itself. Again, past discussions on the issue are a good place to start.
Or, if both circuits are from a single ISP and terminate in the same POP, you can bond them together into a single virtual pipe with MultiLink PPP or perhaps MultiLink Frame Relay if you use WorldCom (they seem to prefer MLFR for some reason).
