You haven't gotten any responses yet so I'll pipe in. I have the older tray-loader iMac. The 233-333 MHz iMac cases are interchangable. In fact, the internal components are intechangeable as well, so you can simply change the CPU card to go from 233 to 333. (Actually not quite true, you have to keep the CRT assembly with the analog video board, as one component, because there were two revisions with different connectors)
Apple seems to keep case designs the same for a model run. So I think it would be worth it to get that 700 MHz iMac (assuming the price is right). That's the fastest G3 iMac Apple made. In fact, when Apple released the G4 iMac, they kept the G3 around for a while longer at the low end (until the eMac arrived), but dropped the top speed back down to 600 MHz. At best, you will be able to swap the motherboard, RAM, and hard drive. Not quite as convenient would be if you had to disassemble and switch the plastics. At worst, the plastics are not quite the same, and you'll have to keep the 'beat up' look, but you'll still have a 700 MHz G3 iMac.
In case you don't have the service manual, here's a site with PDF downloads that may be useful.
http://www.baconeggs.co.uk/manuals/index.htm