that's a nice machine. when pricing it, we should keep in mind what new machines are out there, and how much they cost. a new powermac from apple starts at $1700. older models can be had for less, but these models are still a lot newer than your 533DP.
but the best gauge is the market. I'd post a high price, and if no one buys, then lower it. if you don't want to price too low, then you might want to wait a bit and hope someone comes along who wants one bad enough.
...sorry to go on like that. I'm sure you knew it.
IMHO: $1000, with monitor.