I have a 21.5" iMac (1TB, ATI Radeon HD 4670 version). Love it. The screen is gorgeous. iLife comes bundled free. iMovie '09 is good, so is iPhoto. Though some people like iMovie '06 HD better (you can't download from Apple anymore though, so torrent), mainly because it's timeline editor is similar to Final Cut. But for basic, amateur movie editing, iMovie '09 is sufficient. Final Cut Express is an option, much cheaper than Pro. You can create some really nice looking projects with iMovie, the themes and such included are top notch.
I run Adobe CS3 and Lightroom, and it works great. Also Windows XP via BootCamp (for CAD). If you are going to be doing a lot of graphic intense programs, I would definitely suggest the model I have, or the 27"ers with i7/i9 chips. I have no idea how iMovie and iPhoto run on the base model. Although right now I think Apple is delaying the 27" models because of display issues (flickering, yellow tinting). Haven't heard anything about the 21.5" models.
No clue about PC models. Have heard the Gateway One ZX4800-02 ($699 or so) is a good middle of the road PC. Cheaper option would be the Asus Eee Top ET2002 ($550). Though less powerful. More expensive would be the HP TouchSmart 300 ($800+) or HP Touchsmart 600 ($1,000+).