FWIW, I have an Athlon XP 2000+ which I've been using for video editing for 3 years (was very expensive when I bought it). I bought my first Mac (PowerBook) six months ago, and a week ago I got Dual 1.8 GHz G5 PowerMac and the Apple Production Suite (Final Cut Pro, etc) for video editing. Now realize that I've only had the G5 for a week, and honestly haven't had as much time to use it as I'd like, but right off the bat it feels much faster than the old Athlon 2000+. Rendering and compressing video in particular is much faster, compared to Adobe Premiere on the PC. Honestly though, even if the Mac was slower than the 3 year old PC, I'd consider it an upgrade because Mac OS X is so much better than Windows.
I can't answer your question about games specifically because I don't play computer games (that's what my PS2 is for), but I would imagine that game performance would be somewhat similar, but would depend largely on the graphics card in your Mac. The stock graphics card in the dual 1.8 is not all that great, but you can upgrade it when you buy it or later if you want. I'm using the stock 64 MB 5200, and it's fine for me, but again, I don't play computer games.