You are confusing the Mac Mini with the discontinued iMac G4. The iMac G4 had one internal RAM slot and one user-accessable RAM slot. The new iMac G5 has two user accessable RAM slots and the new Mac Mini has only one RAM slot which is not easily accessable. Depending on who you listen to, it may or may not be easy to snap open the Mini and Apple will probably continue to honor your warranty as long as you don't break anything while upgrading.
Regarding Windows Movie Maker, the current version 2.0 is way waaay waaaaaaay better than the original 1.0 release. It's actually usable now! iMovie has many more features and a nicer fit and finish, but if you just want to splice together some home videos, WMM is probably more than enough. (WMM 2.0 can work with DV, right? I remember that WMM 1.0 only worked with WM videos, not native DV, ick)