I use a dual-2.5 G5 without much complaint. I will say, though, that DRIVE SPEED can often be a more significant factor than processor power, especially when dealing with multiple streams.
I run a 2.5 G5 with fcp 5.1.4 and have never had a problem. It has 2.5 RAM and has been plenty fast for most things. However rendering magic bullet is a bit slow and I would like to increase my RAM, but i think that program is inherently slow.
PM G5 Dual Core 2.0 - 2GB RAM
FC Suite
Abode Suite
Cut several SD feature length projects over the last 2 years. Recently moved to HD from my HVX, and it can do everything that it is made to. HD is obviously more render intensive and slwo to respond at times. I'll be getting a black magic DL card soon so I can preview and down-res. It's been a real workhorse for me. I constantly go between 4 intensive programs at a time.
You can find used DC 2.0 for about $800 I would imagine. Add plenty storage for $500, RAM for $200, BM Card for $500 and you're all set. $2000. (Assuming you have a deck/preview monitor already)