http://www.bridaltips.com/diamond.htm
this is a really good thing to read when looking for a diamond for an engagement ring. There is a lot of really good info in there. I recommend reading it all (I too am soon to be in the market).
also, I guess most people use the same stone for engagement/wedding, but put it on a different ring? That's kind of the vibe I got. I'm sure it's not unusual to just use the same ring though.
edit: it's a tutorial by an independant guy, so it's not biased towards any certain stores.
edit again: I like this guys thinking too. He has a very simple formula to determine how much you should spend:
"Your love for your fiancé has absolutely nothing to do with the price of the diamond engagement ring"