Do you get 640x480 in everything? To fix it, as root, open xf86config. You will see a line with your resolutions in it. At the front of it (I'm guessing yours says resolution "640x480") add your new resolutions - so it should then read "resolution "1024x768" "800x600" "640x480"" minus the 2 quotes at the front and end. Save the file and restart X and you will then be able to change them.
As far as refresh rates go - check your xf86config and make sure that the rates in it are the correct ones for your monitor. Also if these games are slow you will need to download the correct drivers for your graphics card. Although Linux detects the card it installs a 'generic' version of it which only becomes apparent when you play a game like Tuxracer.