I'd guess you'd be paying close to 800 bucks for an imac g4 with those specs. Then again, that's ebay, so you're taking a gamble to begin with there. Similar imac g4s from any reputable web dealer look to be in the 1000 dollar range. I'd say a refurb imac g5 in a heartbeat, but since you already have one, I'd go for the mini, which you could also use as a second computer with the crt for now, and then a wonderful multimedia server if you were to hook it up to a nice widescreen television that you might buy at some point in the future?
I've been tempted to get a imac g4 because that chrome swivel arm is just so darn sexy, but when you consider longevity, any g4 is going to be getting old, real fast. So perhaps just getting one of the first or second revisions on the cheap? Just as a piece of apple history? I found a few 800mhz 15 inchers, with superdrives on ebay for under $550. Geeze that was confusing. Sorry! 😀