Just turn off vertical sync in your display properties to eliminate the factor Grimbar mentions. My monitor incidently does 800x600 and 640x480 at the same video bandwidth which is also the monitor's maximum bandwidth. Limits don't start to kick in until 1024. Your monitor may be the same here so sync rates may not be a factor at all for you.

Interesting figures.. wish I could add more but I need to get a hold of a low end pc myself before I can do much performance testing. I am also observing that my levels are not stressing my dev machine noticably. My main annoyance is build times.