There is, really, a capacity limitation; the amount of RAM in the mk1 means that you start to run out of caching room if the database gets too big. Personally I wouldn't put more than about 40GB in a mk1 - and people with twin 60 setups would want a mk2a because it has 16MB.

In theory, you can upgrade a mk1 to 16MB with another 4 DRAM chips on the underside of the main board and changes to the kernel, but I don't think the player s/w will recognise it...

Hugo