I would start by doing some reading. Calling stores is great, and projector people is excellent, but you can't really beat the reviews you get at a lot of the online sites and the community you get with talking to other owners, vs. a salesperson. I wouldn't rush to buy ANY projector that doesn't have at least a couple of reviews under its belt as the difference between a 'bright' business class projector and one that is more home theater oriented will produce dramatic results in your viewing experience. Most often, if you have true blackout shades, it means that your room is nearly perfect for using a projector day or night. If they are just really dark shades with light flitterring through the sides, you have to be a bit more careful.
Either way, your screen size should be one of the first things you consider. As you already own a projector and screen, you should have a pretty good idea of what YOU like. The general rule is that the screen should be about .66 times the seating distance in width. So, if you sit at 12 feet, a 8 foot WIDE screen is THX recommended ideal. This is a very common seating distance and the screen size still ends up being a bit big when you are not viewing HD material.
Fortunately, with HD discs finally hitting the market, the availability of HD material and true HD movies is finally becoming a reality (with a few tweaks still necessary with HD discs and a format war to play out).
I would not think to recommend a screen size without knowing your seating distance and without knowing your now experienced, personal preference.
Next we pick a projector. The projector should be no less than 1280x720. There will be new models in upcoming months that will be 1920x1080, but pricing is likely to be over $5,000. Not sure if your budget works with that, but it is worth considering if your budget does allow for it. As I said before though, you really want to read reviews on projectors that are out there. In the past 5 years we have seen literally a dozen plus new entries into the home theater market and they are getting better and better every year at ridiculously low price points. That 7 year old $10,000 Runco is pretty easily blown away by $2,000 projectors of today, in almost every area.
The last thing to consider is what screen material that should be used. The gain will be determined by the actual calibrated brightness of the projector and the color will be more of a factor of the room's ambient light.
Besides the very good reviews here, check Projector Central (.com) and some other sites for info and decent professional projector reviews/evaluations as well as tons of user feedback.
Then, come back with questions - keep asking! Put together your real street budget, and don't forget the audio!