You can use two internet connection on your network and get combined speed. I have Cable and DSL - working together. You will need minimum two computers, two routers and one computer has to have two ethernet cards.
This is copy and paste from Speakeasy speed test website: "Download Speed: 6991 kbps (873.9 KB/sec transfer rate)". My cable used to have a bit over 4000 kbps and DSL up to 3000 kbps. Looks like I managed to combined download speed of two internet connections.
If there is an interest, let me know and I'll tell you how to do it!