First question that has to be asked, is what model you have and what CPU - that will determine the whether it's possible issue.
After that, it's only worth it:
IF you have 64-bit software (you may be in for a nasty round of upgrades if you have 32-bit software that won't run),
IF you have ALL 64-bit compliant hardware (you could find yourself needing a new printer, scanner, etc. if yours has no 64-bit support), and
IF you have 4G or or more of memory (it makes no sense to use 64-bit software with less RAM than 4G).
I have a Dimension desktop which I would like to convert to 64 bit, 1) is it worth it 2) How do I do it?
If you mean upgrading to a 64-bit OS, you'll probably want 2gb RAM, for starters. And, of course, you need a 64-bit CPU; if you don't have one of those, you're better off buying a new Dell.