I can average 21.7mph for a 20 mile time trial (00:55:15), on my 2001 Rockhopper with 1.25" fatboy slicks, no lock out on shock. My bike weighs 25.25 lbs. I'm planning on buying a Cervelo 2012 r3, 16.4 lbs estimated weight. I also plan on dropping 4-6 lbs, so my total weight would be about 14 pounds less. Does anyone have any idea what my average speed would be for a 20 mile time trial in good conditions, 10 mph max wind?
You might want to play around with Bike Calculator a little, and you could find a reasonable estimate for your question.
Regarding the new bike purchase, shop around carefully. You're already past what is considered the clearance season for 2012 models at many traditional brick-and-mortar bicycle retailers. Prime time is August/September of each year. That's when many retailers start their clearance sales.