Replacing a 4x40W amp with the 6X40W is very simple. You unscrew two bolts, unplug the wiring harness, cut a small tab off the 6x40W amp connector with an exacto knife and install in the reverse order. You only need the extra 2 channels if you want to run rear speakers off the amp instead of the HU (HU supplies 15W to the rears).
There are a lot of amps on eBay, but most of them are 4x40W. I believe there are two versions of the 6x40W amp. There is a ton of info on this if you do a search here and on google.
