Well, be forewarned that the 17" iMac is going to be competition by the time you add everything.
But, if you want to go low-end:
(1) There's no reason to get the 1.42 GHz version; upgrade the 1.25 GHz version to an 80GB drive, if you want, and save $50. Or stay with 40GB and save $100. So, let's say you get the cheapest one.
Price so far: $499.
(2) Spend the money to upgrade to 512MB. $75 is worth it.
Price so far: $574.
(3) If you don't mind a CRT, you can get a 17" for $99. Maybe ~$300 for an LCD.
Price so far: $673 (if you get the $99 CRT).
(4) You can get a decent KB/mouse combo for $30, maybe less.
Price so far: $703.
So, I'd say a bit over $700 for the cheapest usable configuration. $1000 or so for some upgrades.