The slot is a PIECE OF PISS to take out. You just squeeze the leavers, and yank!it is possible to upgrade a Pentium 3, you just need to know when one you are useing, are you using a slot or a socket? if you are useing a slot, there more of a pain in the ass to get out. if its a socket its pretty easy, take off the clamp on the heatsink and then there will be a little lever, pull that outword alittle and then up.
if you want to know how much you can upgrade it too, you will have to find out what mother board you are using. after you have found out what motherboard it is, look up online its maximum cpu capacity and then you can upgrade it, as long as you dont surpass the maximum capacity.