Depending on your video card there should be a gamma setting slide bar under settings, advanced then the adapter name. You can use this to change the baseline gamma for all games to be brighter.
1). Set it so your worst game is just right.
2). Then go into the games that were ok and darken them down.
8 years is getting along there and the eelctron gun in the tube is probably wearing out, but if you can squeeze another year out of it you might as well. At the rate display prices are dropping, in another 11 months they will be paying us to take them away...
Some games like Tribes II just come out too dark even with the internal adjustment. Gamma "warps" the way the brightness and contrast signals are interpreted.