[Arakiel]

Makinus -

The OS isn't going to matter with that machine. Your problem is going to be the video card. 256 shared on a 1ghz machine is likely to be slow as hell for modern games no matter what OS you have on it. I'm in serious doubt if you could even play Doom 3 at all. Evil Genium i'm not familiar with but I believe it is 3d, yes? The shared video memory is gong to slow you down with any 3d game.

The above suggestions by Fyron and aiken will help free up resources for the computer and make surfing and emailing faster, but for gaming the only thing that is going to help is a better video card and more memory.
Also, given the above specs, a FULLY PATCHED 98se is going to be your best bet, XP will run dog slow on 1ghz/256mb. (editing: Although if you turn of all the fancy visual bells and whistles in XP it might be "ok"....still think 98se would be faster though)

You might want to consider Linux instead to get even better speed (pretty much rules out games though, games under Wine generally stink and there arent many native Linux games) If you do go Linux I would stay away from KDE and Gnome for a desktop environment, they can be hoggish on resources. Look for something more like Fluxbox which is nice and lightweight.