I wanted to upgrade the RAM & increase the size of the HD in my Mac Mini. I needed to keep a Snow Leopard partition (to run Rosetta) and download and install Lion on another larger partition. I swapped the original 80Gb HD for a 7200 rpm 500 Gb HD and while I had the Mac Mini open max out the RAM to 8 Gb.
I downloaded ifixit's iPad app and used there step by step guides on my iPad, which was useful for enlarging the step by step photos. It made it easy to open, disassemble and install the new parts. The Putty Knife and toolkit I bought were invaluable in successfully upgrading my Mac. The upgrade went perfectly, and I had Time Machine reinstalling my files in no time at all. I've been using my mac for about 6 weeks, with no problems to report.
Don't rush, study the photos in the guides and make sure your in a well lit room, on a clear desk/table.