Can I have 64bit Linux installed on my Desktop PC?
Yes. Starting with SL5, this is fully supported, and quite a few 64bit desktops are in use already. We actually encourage users to take this step into the future. All it requires is a reasonably new PC (purchased summer 2004 or later).
Where reasonably new means your PC is at least a Dell Precision 370 ("satyr", first purchased in Summer 2004) or a later model (Precision 380 "satyr", Precision 390 "oreade", Precision T3400 "pepe", ...).
Simply ask for the 64bit variant when requesting the update of your desktop. Converting a PC to 64bit is actually the same procedure as upgrading it from a previous SL release.