You won't come close to the specs of a MacBook pro, but you can build a respectable core 2 duo machine with similar specs to the current generation Mac minis. I have just completed my iMac g4 20" all in one mod by using the LCD controller board and wiring from a 20" apple cinema display and an ECX form factor motherboard with a 2.53 ghz p8700 core 2 duo processor. The cost of the "computer" including board, ram, fan, CPU etc was around $600 total. This includes the 32gb compact flash card that I'm using as a HD. The power bricks are external (because of space and heat issues).
If you already have the iMac g4 chassis you don have to worry about this but a non-working 20" iMac g4 with a likely working screen can probably be purchased on eBay or around $100. Then I'd look for a cinema display with a cracked screen or faulty inverter (if the iMacs LCD is broken, the apple cinema display LCD can replace it). I got my cinema display for around $60 with a faulty inverter. Even adding in cables, supplies, blu ray drive, it's still likely to be under $1000. Is this a lot to pay for a 20" core 2 duo machine that you still need to put together yourself - absolutely!! But from the sound of it you're like me and believe that the iMac g4 was represents the best designed desktop ever.
Here is the link to my blog with pics and more details
http://dremmeljunkie.blogspot.com/2011/05/20-all-in-one-mod-it-works.html