I just bought a Casio Cassiopeia PA-2400U and accessories off of e-bay for $50.00. I can't seem to find any programs that will work with this set-up. I am most interested in games. Also, can I upgrade to Pocket PC 2000?

Firast off let me say you got a pretty good deal. Yes it's only 80mhz with a greyscale screen but it was solid as a rock with great battery life.
As for the software there are still a few sites out there. You could start at cesoft.com also conduits still makes software like an MP3 player and some network tools. I think Ruksun makes some instant messengers as well. If you can't find what you're looking for let me know, I still have tons of CE software that I use on my older PDA's like the Casio E-100 and the old Jornada's.
Oh, and no, I'm afraid you can't put Pocket PC on it. Sorry.