You can connect 2 ISPs to your 2811. BGP is the best way to multihome to ISPs and load share but this may not be an option for you depending on factors involved. You can load share (i’m not using the term load balance on purpose) outbound traffic with two default routes, one to each ISP, configured in your router. This will load share traffic on a per destination basis. Meaning traffic to a single destination will all flow over the same ISP connection.
You also need to think about the return traffic from the ISP to you. You can control this by using network address translation (NAT) to translate your internal IP addresses to an IP address assigned to you from the ISP which should ensure that traffic returns on the same connection it went out from.
Here is a link to the modules you can install in your 2811, http://cisco.com/en/US/products/ps5854/products_relevant_interfaces_and_modules.html.
I’m guessing your ISP is providing you with an 8Mbps Ethernet connection? Is the bandwidth limit in both directions, upstream and downstream? More than likely your provider will give you a 10 or 100Mbps connection and use some form of rate limiting to maintain the bandwidth limit of 8Mbps. If you think your outbound traffic will exceed 8Mbps you can do the same to better manage outbound traffic.