I am a bit loathe to post shots of 640 X 480 interlaced on a Windows desktop, mainly because it's not very good! The desktop does not lend itself to interlacing bacause it often has single-pixel lines, which interlacing causes to flicker at 30Hz.
This is not really relevant for gaming as you would never have any need to run at the desktop resolution in MAME. For Windows games, including 3D, interlace produces a surprisingly good picture though.
640 X 288 is fine for the desktop but not entirely useable. XP auto-pan gives the best of both worlds, 800 X 600 desktop on a non-interlaced 640 X 288 monitor resolution.
As far as MAME screenshots goes, I am not sure of the relevance because the quality of the shot is entirely down to the cameras capability of reproducing th monitor picture and the quality of the monitor itself (ie not old and shagged out!) and the cable and connection method including grounding (ie video bandpass). The card, when used in a native res mode, will simply map the game onto the screen with perfect quality given that all the other factors are well sorted.