What are your needs?
I would possibly try to avoid the imac, as if they update the line the prices can only plummet.
If you just need basic internet and email, get an older g4. I have a dual 450 that I bought for 50 bucks that is fine on the internet and email. Nothing special, but for 50 bucks you can't go wrong.
Here on craigslist there is a guy selling a couple of power mac g4 400 MHZ (single processor) for 70 bucks.
If your needs are greater, then perhaps a G5. No update to mac pros should let them keep their value.