What tires will you be using? If you go larger than 32c I suggest using a wider rim than what people have suggested here.
What I used for a number of years was Mavic CXP33 rims and Shimano hubs.
You get them pretty cheap on the net from places like Colorado Cyclist. You have many choices for tires. Continental has a brand spanking new 28c tire that has Vectran and weighs 9 or 10 ounces. Very light for the size. I used to
use Rivendell Ruffy Tuffy 27c tires and with the Mavic rims it's pretty quick.
However... while all this will help, I have been down this road before. The frame has some give built into it that eats some of your energy. It just wasn't
intended to go fast.
IMHO... look at used road bikes, preferably in the Sport category cause they will take a 28c tire. One example of the breed is the Surly Pacer, another is the Gunnar Sport. They've always been around, and they're not hard to find... my first road bike was a Schwinn varisty it's one.