Replacing the hard drive I'm these machines is quite a job, so please make sure you read the guide before starting on it.
[guide|731|Replacing hard drive]
As to the actual question, this model supports at least up to 2TB on software level. The main problem is that it still needs PATA (not the now common SATA) disk drives. They are luckily still made, but are harder to obtain. You can fit in a model up to 9.5mm in height.
As far as I'm aware this limits your choices to 320gb in regular mechanical hard drive (I haven't seen any larger models up for sale), or in SSD a max of 480GB, like for instance this [http://eshop.macsales.com/item/OWC/SSDMLP480/|OWC].