Hi there,
Does it say anything else in addition to the screen resolution of 800x600 (like perhaps 16bit colors or another x number after)?
If running windows - (just in case you don't know) - the way to change your resolution (including color depth - the amount of available colors), is by right-clicking on your desktop (on a blank area) and then Properties (or you can go to Control Panel -> Display Settings).
Then click the 'Settings' tab - you should see settings for the resolution (800x600 / 1024x768 etc etc).. but also the Color Depth (16bit / 32bit - other options maybe).
If you are already set at 800x600 and that's what the program is asking for - maybe try changing the Color Depth.
Otherwise - I'm afraid you are best off contacting the Casino Support.
Hope that helps.
Cheers,
Geno/CC