That depends on the MacBook model.
The white pre-Unibody MBs can handle any 2.5" S-ATA HDD with a maximum height of 9.5mm, so that's 640GB.
The Unibody MacBooks (aluminium and white) can handle any 2.5" S-ATA HDD with a height of up to 12.5mm, so that's 1TB.
The maximum RAM can be from 2GB to 8GB.
Look here to see what the maximum RAM for your specific model is.
Also MRoogle will find you one or two thousand threads about the same issue.
There is nothing else you can upgrade, besides the HDD and RAM, but you can use an
SSD instead of an
HDD to speed things up a bit.