Upgrading computer Windows 7 from 32bit to 64bit

I fear the logical answer may be to simply go out and buy a newer computer!!  However, I have a nicely running Dell Inspiron 530 as well as an Optiplex 760.     Both are using 4Gb of RAM (but with only 3.2 Usable). Both are 32-bit systems, and I would like to add more RAM to at least one as they are both fairly slow machines!, but that means going to a 64-bit upgrade as they apparently can't handle more RAM as they are.

So the query is - am I just making work for myself or is such an upgrade fairly easy please?