Just get the largest SATA II drive you can get your hands on, watch the physical size though..
think is something like 9.5mm anything bigger might not fit the HDD cradle.
I'd recommend a Western Digital 5400RPM 500GB blue drive, about the largest you can get for the money.
some have said the 7200RPM do work, but concerns about heat, but me I don't think that'd be an issue... But performance wise, 5400RPM - 7200RPM you'd not notice any difference TBH.
If you're mr money bags, then SSD might be worth a look, but honestly, a 500GB SATA II 5400RPM will be just fine..
You'll have about 465GB usable, so not bad really....