When I bought my 24" iMac back in 2007, I thought a 250GB hard drive would fit my needs for a while. How wrong can one be. Now 2 years later, this drive is almost filled.
So which options did I have… adding a external firewire 800 or swapping the internal harddisk.
Both options have their advantages, an external drive is easy, an internal drive is faster.
I choose to go for the cheaper and faster internal option.
There are some websites describing how to open an iMac. One about a 20 '' white Intel iMac, and one about the alu iMac.
A 24" white iMac isn't assembled the same way as a 20" version described on the first site. Combining the to disassemble instructions I was able to perform the trick.
Here some pictures:
Patient is prepped
After removing 4 screws the front is lifted easily
TFT panel removed
Her's the old drive with the temperature sensor still attached
So now I have a nice 250GB Seagate Barracuda 7200.9 SATA2 drive for sale, let me know if your interested 
If you have any questions just let me know.