If you are in windowed fullscreen mode, it may well work better in 800x600 than 640x480 if thats your actual windows desktop resolution. In full screen exclusive mode the lower the res, the better the performance.
Whilst I always thought that DBPro would be set to 16bit color as default, it seems it may be 32bit, as some people running some simple example programs in default res complain of "your card does not support 32bit color" style errors.
On almost every graphics card, lower color resolution equals faster performance. Only rarely have I found cards thst prefer higher modes.
