OK, everything has arrived and everything is installed.
Found used memory that looked exactly like the one that's supposed to work in MPC-4000. $20 for 512 MB RAM and it works perfect.
Replaced the fan with the fan meantioned in another thread here. The difference is HUGE. This one is really, really quiet. Every MPC-4000 should do that. I check regularly if it's blowing, and yes, indeed it is.

As you all know I also replaced the hard drive. I thought I ordered a 120GB laptop drive, but it was 160GB laptop drive (Western Digital). It is definitely quieter than the one before, but to be honest the fan used to make most of the noise. Now it is the harddrive. I think it sounds a bit more than it should, but that's because the MPC is a a huge resonating box.

Still, my MPC is very quiet now and the option "NO SPIN" is still there.

About hard drive sizes...yes, there is a limit. My 160GB drive is formatted as 127GB, so anything bigger than 160GB is useless. 120GB (=112GB formatted) is the way to go if you want to use the whole drive.