no - not really.
or better - not with apple hardware.
sure - you can take everything out of the iMac and replace it with newer parts. but everything must go.
even dremel junkie didn't go all the way with his mod's.
you would need a new monitor (if you have a 17" with an aspect ratio of 16:10 - like the hanns g. lcd's)
a new mini itx mainboard - they will fit into the iMac - but watch out, there are i7 mini itx boards on the market, the cpu's are expensive and the main problem with them is the heat they produce.
i have a "simple" core2duo mini itx board for my iMac project.
it's smaller than the usual mini itx - that way i have a little more space for all the cables i have to put in, my board has 2 vga and 1 hdmi output, but one vga is needed to connect the hanns g.
it's way to much work to explain it that fast
the good part is - my system (10.6.3)is up and running - i "only" have to build everything in the iMac
and one thing - the price of the iMac depends on the parts you want to use. but maybe ther would be another option with a MacBook logicboard - but that would be not easier to build in
What is your machine's model number, it will help me find what parts you can use in greater detail? If you still want it to be a mac, I would recommend the following:
Change the original hard drive with a 7200Rpm IDE drive (which you can get in sizes up to 400GB)
upgrade the ram, Apple says that the max is 1GB but it will actually will support 2GB
put in a new DVD burning drive, you need an IDE drive and you can get up to a 24X DVD burner.
You can install an airport wireless card, depending on your model, you can get either an 802.11b AirPort card or an 802.11g AirPort Extreme card.
I don't think that you can swap in an airport extreme card (ie 802.11g), this mac was designed for the original airport card (802.11b). anyways, doesn't make sense since the standard is now 802.11n. the better route is to connect the ethernet port to a wireless bridge (eg Dlink DAP 1360).
i've maxed out the RAM to 1GB (using a part from a burn out ibook :) .
as for the HDD, I've swapped in a SSD of IDE kind, with the appropriate dongle adapter. This is the biggest upgrade in terms of speed, highly recommended. i got a 48GB drive rather cheaply (forgot the details now).
Stuck with Tiger since the graphics card on the machine isn't very strong. boot up time rivals recent MBPs. for casual internet (no flash, no video), it's quite good.
all of this for a machine design that is timeless!!!!
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