The MX series were horrifying... those were around when I was still in High School, and they were... unbearable.
Just junk the system out, saving what little you can. Take 300 bucks and build a very low key budget system. If you use your current case, and throw in some really cheap junky parts, you will find you have a pretty decent system, especially if you are running good old Win98.
I will post a parts list for you in a little while. I think I might be able to go to sleep now... Give me a few hours kk?
As for the rest of the recommendations. You won't be able to put a DX9 card into that relic. More RAM won't save that PC. Nothing can save a PC that is THAT old (presumably) It is Game Over, you lose, good bye, shot in face, put into grinder, toodles, aufwiedersehen for that poor thing!
Your old games will run perfectly fine on a slight "newer" system. Even an extreme budget system will be 1000000000x better than what you presumably have right now.
Like I said, give me a few hours to sleep. My back incision started bleeding randomly earlier (just a little amount, but that means I need to take it easy on it and lie down for a while) I put on a surgical sponge and a new steristrip, and a water cover over it. I think it should mend up with the iodine and neosporin treatment in about 8-10 hours. I am about 75% back to my normal self. In 3 weeks, I can lift more than 10 lbs! You have no idea how restrictive I feel not being able to do more than that. I can drive as of Thursday (10 days was the min), and as of Monday, I am totally cleared for all activity minus the 10 lbs limit and excessive twisting of the back. Can't wait to drive my friggin new car!!! OMFG!_!_$(%@!#*$&!@# Ok, I am going to bed. /end diagnosis