Rogue

What is your specs? My old pc that has pretty bad specs (2 gb of ram and low processing power) can run custom maps and mods with little lag. I am not sure about the story mode though. I think I tried it once and it had a lot of lag but it was a long time ago.

I am giving you this info based on experience that I still have today

Easy: You can't. Turn down all graphical settings and hope for the best.

This guide should help: http://forums.steampowered.com/forums/showthread.php?t=1034100

It all makes sense, since that game was all I played when I first had this computer w/out a graphics card.