Is this your and your girlfriend's first foray into video (DV) editing? The only reason I'm asking is the "using iMovie" part of your post. To be frank, a dual-gig G4 for using iMovie is a bit excessive or overkill. Most of my iMovie stuff is done on a G3/400 iMac or iBook/700.
Depending on your budget, an eMac would be fine or, if you wanted to burn DVDs, a flat panel iMac--the 17" is especially nice. Load as much RAM as you can afford, and an external FireWire drive for extra storage is a nice perk, but not a requirement.
For a camera, make sure it has optical image stabilization, not digital. Sonys or Canons are good choices, and be prepared to spend around $1000 for it. To me, investing in a good camera should carry the most weight in your budget.