If you are looking at upgrading RAM, the maximum can depend on what bit OS you are running (32bit or 64bit). To check what you are running goto Start >> in search type without quotes "System" >> in the search results find "System" which should be under the control panel catagory (the icon for it is a blue computer monitor with a tick inside it), it will tell you the bit in there.
As for your friend, he is most likley running a 32bit system which can take a maximum of 4GB RAM, but like mine, will only display as 3.25GB in some places. 32bit systems can only physically USE only 3.25GB even if you install 4GB. 64 bit systems on the other hand can have TB's of RAM, and will display and physically use all of it. Wierd I know, but that is the way t'was all made to work.
Hope that helped