Depends on what you consider decently and what parts of iLife are important to you.
I have a new iBook 1ghz with 384 MB of RAM. I can run iTunes with no prob at all. iMovie is pretty snappy but could improve. Garageband, again--adequate, pretty snappy, but still could improve a bit. Sometimes when I get up to 5 tracks, the playback can start to lag. But other than that I really can't complain. iPhoto is pretty snappy, much better than the last version, for sure. My library is around 5000 3-7 megapixel photos
iDVD sucks. It takes forever to encode--12 hours last time. I have to keep my movies under 1 hour or they don't ever come out. Sometimes the menus are scrambled.
The bottom line: If you're looking to do movies and stuff with iMovie and iDVD, get as much speed as you possibly can afford.