I would suspect that these machines can accept drives up to 128 GB, like they're contemporary PC counterparts (I'm not 100% certain of this, but it seems logical that it would EDIT: confirmed; ATA-2 (which I believe the P63x uses), is limited to 128 GB). However, you have to take into consideration the fact that the OS must be within the first 8 GB of a given disk, or else it won't boot.
Then there's the issue of having the OS properly recognizing such a large drive (Mac OS 8.1 I think will work better than anything else, since it supports HFS Extended partitions (that are, I think, larger than 4 GB); however, you cannot boot any 68k Mac from an HFS Extended partition- you must have the OS installed on an HFS Standard partition instead. Of course, this doesn't prevent you from actually using HFS Extended on 68k Macs, provided you're running OS 8.1). I don't know if you can like have both a Standard and Extended partition on the same disk, though.
As for the RAM, I have no idea.
c