SQL 2005 doesn't have resource Pool management like SQL 2008, so no for doing anything per user.
as far as limiting to a processor you can use sp_configure and affinity mask to limit to a particular processor. I would read up on this topic and practice it as this is not something to do on a production system without a lot of testing
As far as memory consumption you can limit the memory under the SQL instanced Server Properties | Memory | and configure the maximum amount of memory you would like to allow SQL to use.
Once again this should be tested in a dev environment as limiting the amount of SQL memory will impact SQL in almost every way, more is normally better not less. that being said I understand everyone has thier limitations they must opperate under, but be very careful adjusting any of these things as you could critically limit your server.