General hints gleaned from watching this newsgroup too closely over the past year:
1. Use S-video, not composite (RCA) output. Makes a world of difference.
2. Geforce cards are notoriously poor for their TV Output. You may not ever get it just the way you want it. If all else fails try ATI (cards made by ATI, not just w/ an ATI chipset) or Matrox cards instead
3. If you are using hardware stretch through windows make sure you set it up for Sharp graphics effect (in a MAME frontend look in the same place you select fake scanline choices for PC monitors). This may cause the games to slow down considerably if you have a slow PC.
On my S-Vid TV cabinet it has taken me a long time to get things just right. You will have to do plenty of tinkering with your TV settings, the software settings for TV output in Windows and the brightness settings within MAME to get your games to look proper. Even then, unless you are willing to change your TV and card settings manually every time you boot a game, there will still be some games that just don't look right.
Quick example: Bump 'N Jump on my computer is almost impossible to see the yellow score information while playing. If I adjust everything to allow this to look right then 95% of the other games are too dark and the colors are washed out.