Presonal recommendation is to go with what fits the girl. Go to a jeweler and tell them what you're thinking about and let them help guide you through with their expertise. Have a budget in mind, but be willing to step outside that if needed. Getting something your girl will like is the main thing. In our case I got her a nice solitaire and then for the wedding we got it remounted into a very nice band in lieu of going with a traditional add on ring or wrap. Bigger and more expensive doesn't mean anything, it's the idea that you picked out a single stone and band that you felt would best suit her.
As far as the engagement, I'd recommend not doing it on the weekend of her dad's party simply due to the fact not only could it steal his thunder, but also her's as well with everyone celebrating his accomplishment, her's might get a little less notice than my be warranted. There are always more oppotunities to do the actual proposal. Is there any significant dates you celebrate such as when you met, etc? If so, try that date. Another thing is that it can be construed as tacky if you try to give the ring as her birthday present. Some girls don't mind, but some take offense like you're trying to kill two birds with one stone figurateively speaking.
Rick
