WSLee,
I did loads of research before I got my wife an engagement ring. I read in a diamond book somewhere that, notable difference in size is observed every 0.25 carats. While I agree with some of the comments above whereby you should try to get a ring which is 0.48 or .095 carats, this ring will be really difficult to find as there will be a significant price jump for every half a carat. I'm very sure that the retailers will try to sell as expensive a ring as they can.
With regards to colour, they are D, E, F, G. D being the "whitest". Anything with H or less will already be "yellowish"
Always try to get a diamond with a perfect cut as maximum light refraction will cause the ring to "bling" more...
Clarity of ring is not of upmost importance as no one will be walking around with a jewelers magnifying glass. VS 1 or VS 2 will suffice. FL or flawless being on the top of the tree, followed by VVS 1 which is very very slight intrusion, VVS 2, VS 1, VS 2 and SI which is Slight intrusion.
I do hope this helps...I always tell my buddies to buy what you can afford and then make for for that one carat or more later in life. Weddings are an expensive affair and I feel for you bro...but this is something that most of us HAVE to do as we want to honour our parents, both on the side of the bride and the groom. If I had a choice, my wife and I would have forgone this whole shenanigan and used the money to buy property.
Well...good luck and congrats
DC