Wow, nice machine!
Unfortunately, I don't have much experience with gaming as most of my time is spent on business machines etc but I'll have a go!
A few things to consider.
Have you installed all the mobo drivers on the system, that came on the CD?
Is the OS up-to-date with the latest patches.
4 gig of ram is nice, but XP or your mobo may not recognise all of it. I would suggest as a testing excersise, remove 1 stick of ram at a time and see if anything works better e.g 4gb -> 3gb -> 2gb ->1gb. Also check the motherboard manual to see maximum ram.
Also try running the games with only one graphics card installed after this, the process of elimination shoud identify what is the problem.
As for empire earth II, I don't know the game, but check this link out:
http://www.empireearth2.com/uk/downloads.html
There's an update for the game and an XP game advisor that might give you more help.
Good luck - don't follow my signature advice (below)