I have a 1024x768 LCD screen which I have connected to a KVM and then to a mac via a 10 switch vga-dongly-adapter..

Out of curiosity I tried using 512x384 mode and it came out crystal clear on the LCD screen. The other great thing is it's a simple multiple in both directions so doesn't look stretched.

The good thing is it means I don't have to find more VRAM to have more colours, just kick down to 512x384!