the 24" are ok ,
but a little tip avoid by all means the 08 model with the optional GeForce 8800 GS
(ignore the tip if you got enough cash to replace that card on a regular basis )
better for the wallet .. settle for the 08 model with the basic spec which was the ATI Radeon HD 2600 PRO instead if you plan to use the iMac on your desk instead of dragging it all the time to the apple store , as a side note the 08 models according apple only support up to 4 GB ram , but actually take 6 GB 1x 2GB module and 1x 4 gb module 🙂
i personally prefer the white 24" because of the matte display, despite the GeForce 7300GT/ 7600 GT but at least these cards dont fail that often