If it isn’t broke don’t fix it. The upgrade from Windows XP to 7 will cost come cash, quite a bit depending on which version you go with.
I’ve been using Windows 7 since it was in beta and I’ve been quite happy with it. Now my computer is much newer that 2002. My desktop also has 2 Gigs of RAM, and it’s a little slow if I open to many apps, but that’s because I run out of RAM. My laptop has 3 Gigs of RAM and doesn’t really have the same problems.
The big thing to check before you think about upgrading would be to ensure that all your applications run under Windows 7. Your computers will be old enough that you won’t be able to use the Windows XP virtual machine which comes with Windows 7 (Ultimate and Business only I think) as your CPUs won’t have the Virutalization code in them.
You may want to look into replacing the machines with newer machines. The cost of the software upgrade may be close to the cost of a new computer, and the new computer will come with Windows 7, or at least a free upgrade to it.
The only person who can decide if the upgrade is right for you, is you.