Having multiple user accounts doesn't affect utilization of system resources, including RAM, unless multiple users are logged in at the same time. During login it may take a few nanoseconds longer to go through the list until your username is found, but nothing noticeable. Over time a system does tend to slow down, though, as a consequence of use...accumulation of installed programs, startup tasks, increased hard drive fragmentation, etc. That's why it's good to perform regular maintenance, such as checking msconfig for unnecessary programs, defragmenting, and clearing out junk files with the likes of CCleaner.

John