From what you described, what you want is to sell the MacPro, buy one of these new Macbooks coming out tomorrow (supposedly going to have better graphics) and then buy a used PowerMac (you can get one for $50-100 that will do browsing, iTunes, etc. just fine in Tiger) and keep that on your desk. If you monitor has two inputs, you can even dock it up when you need something faster, but you can't beat a $50 browsing/music server machine. You could even upgrade it for a couple of hundred to something you'd never notice the difference on. I've got around $1000 in this PowerMac and it's 1.8GHz 7448 G4, 1.5GB ram, two 500GB hard drives, ATI 9800 Pro, USB 2.0 and takes up no desk space (sits in tower nook under the desk). For browsing, etc., it 'feels' just as fast as my 2.8GHz Dual Core AMD PC sitting to my right (L-shaped desk) and it runs 24/7 without issue and also serves as a server for my whole house audio system with iTunes8.
I'm planning on getting a Macbook soon (possibly this week with the new one coming out) and I'll have dock capability for that and be able to use it anywhere in the house (including connected to my 93" screen and projector home theater downstairs, on my back porch drinking coffee in the morning and with me on trips or sitting in bed if I wanted to (not sure why I would; doesn't sound comfortable to me) and most importantly as a portable studio for recording music (so I don't have to junk up my living room where my Roland digital piano is with computer equipment; I'll just connect it while recording and disconnect it when I'm done). I can then even network back the finished results to my PowerMac or PC for storage, all wireless (I have two 802.11N networks running in my house so G-devices like my iPod Touch don't slow the main N one down).
The point is you could have a somewhat used desktop (there's also plenty of used iMacs and PowerMac G5s out there to consider) plus a brand new Macbook for the price of your MacPro or less. Heck, you could have TWO Macbooks even for the price of a MacPro and use one as a desktop and the other as a laptop if that's your thing.