Welcome Bill, and happy belated birthday! Your wife has no idea the problems she has just caused in your marriage!

Alright, seriously now. Let's see.
The differences between 2002 and 2004. Basically, they just enhanced a bunch of stuff. Scenery, planes and weather are a bit more enhanced, etc. They also added taxiway signs.
I'm sure there are gobs of more changes internally, but as far as visual aspects, that's about it. (And they did a very good job!)
There are also a few more ATC options, such as the ability to amend a flight plan in flight, or change altitudes.
The difference between 2k2 Standard and Pro? The main difference is the default fleet. In the Pro version you get a few more airplanes and also a 3D modelling tool. But to be honest, I never fly the default planes anyway, so even in 2k4 (or FS9 as you'll often hear it called) that's just wasted space to me. As for the 3D modelling, you can download that for free anyway, so it's not like you're missing out on much.
If you're trying to upgrade 2k2 via downloads, you'll probably be able to get most of it. Taxiway signs aren't going to be easy to find, but as far as planes are concerned, you'll find just about anything you're looking for.
All in all, especially considering that you're essentially getting it free, I'd say hang on to 2k2. Down the line if you feel the need to upgrade, you can always do that as well.
FS9 is a bit strenuous on the computer though, and unless you consider your system to be medium to high end, it might not be that great for you.
Welcome again and good luck!