And the forums have already helped immemsely

I just bought a Trek 6500 disc without doing a ton of research, but still did a fair amount. After coming here though, read all about the Manitou forks. After talking to the LBS I got it from, they let me return the bike in exchange for the Cannondale F600. I still have my Trek 3700 also, but believe I may be spending too much money on a bike that doesn't have a good platform to start with? I've just invested about $500 in parts for it, including a Marzocchi fork, Avid BB7 front disc set, new brake levers to accomodate the set, as well as cables, new deore shifters since the brake levers won't hold my hold ones, and a new shimano lx cassette, since I couldn't find any standalone trigger shifters for my old 7 speed. Now, I'm curious to see if I'll need a new chain for the cassette that's coming in, and if I'll need a new rear derailleur as well. I guess it depends on the measurements of my old cassette?

I think I've managed to find yet another money pit