just bought one myself last week, this is everything I learned:
would avoid tiffany / brands - a diamond is something that is marked up pretty heavily anyway, let alone slapping a brand name ontop as well, you will loose a fair chunk of value to the shop straight away
wouldnt bother going above vvs1 for clarity, d-j for colour,
better to get a good balance of everything rather than just going for size, and above vs1 you wouldnt be able to see anything wrong anyway so not really worth paying the extra for
anything below 0.4c seemed pretty small, 2c seems massive
platinum is the best material for the band, although palladium is just as good really and cheaper, white gold seems to get knacked quickly
dont go any lower than "good" on the cut scale, it affects the sparkleynessness
dont buy one unless it comes with a GIA certificate
I went to brighton which has many many shops, so can get a fair idea of pricing / what you get for your money pretty easily