It probably isn't worth it for you to go with a 64 ,bit OS, with the uses you describe. 4GB or more of RAM is normally utilized by very,very large files in Photoshop, or hi-def video >> encoding<<, and extreme multi-tasking. If you can pull up task manager while you're working, see exactly how much RAM is being used.
Sometimes in pre-built computers such as your Dell, as someone else mentioned, drivers may not be available for OS versions that were not available at the time the PC was sold.
In other words, if your PC shipped with XP, then XP 32 bit, and possibly XP 64 bit drivers may be all you'll find. If Vista drivers are available, they >>might << work, but it's sort of a crap shoot. You can also download M$'s "Windows 7 upgrade adviser" software,
http://www.microsoft.com/downloads/...90-7659-4BD9-9E51-2497C146AF15&displaylang=en that will tell you if drivers are, or will be available for your particular machine.
That said, I wouldn't build a new PC without a 64 bit OS. However, I'd think long and hard before I decided to retrofit my older machines with one.