You did not indicate if you're using NAT or BGP. You need also to consider how your "return" traffic will be balanced.
However, assuming that you use a couple of default routes, one to each ISP:
1. Assign 3 more subnet addreses to your 512K interface as secondaries (you'll end up with 4 routes to that serial interface).
2. Add 3 more static default routes each destined to IP address within one of these subnets. (you'll end up with 5 static default routes in your config)
2. Change the number of maximum paths from default (4) to 5 using the maximum-path global command. (you'll end up with five routes in your routing table: 1 to the 128K and 4 to the 512K).
Example of these commands:
maximum-path 5
in s0
bandwidth 128
ip add 10.10.10.1 255.255.255.0
in s1
bandwidth 512
ip address 1.1.1.1 255.255.255.0
ip address 1.1.2.1 255.255.255.0 secondary
ip address 1.1.3.1 255.255.255.0 secondary
ip address 1.1.4.1 255.255.255.0 secondary
ip route 0.0.0.0 0.0.0.0 serial0
ip route 0.0.0.0 0.0.0.0 1.1.1.2
ip route 0.0.0.0 0.0.0.0 1.1.2.2
ip route 0.0.0.0 0.0.0.0 1.1.3.2
ip route 0.0.0.0 0.0.0.0 1.1.4.2
