Turboem1 - it'll generally work fine to have 28c tires on narrow road rims, as newbojeff is doing right now.
For clearance, you can just look at how much space is above the 25c tires and get a sense of whether a larger tire will fit. Most racing-design road bikes won't take larger than 28, and often will max out at 25c. But you should be able to get some sense of clearance under the frame and brakes by just eyeballing.
There was a recent thread on this topic, and you should be able to find it using the search. I think it was specifically about Mavic Open Pro, the standard narrow racing rim.