A lot of people do - it's often cheaper to buy an off the shelf machine and upgrade the ram yourself if the rest of the parts fit what you want.
IIRC the warrentee void stickers on a computer are not legally binding, if you do a google I think there were several court cases where it was decided that due to the nature of the PC it was expected that they could/would be opened at some point and thus the act of opening them would not itself be deemed to void the warrantee (in fact some companies will ask you to open the machine during fault finding/send out the replacement part for you to fit if it's something simple like a drive).
That is going completely from memory, so it could be wrong.