Without even a prototype — like Microsoft‘s (MSFT) — to look at, 21% of 3,100 respondents in a RBC Capital/ChangeWave survey said they’d be interested in buying an Apple tablet computer in the $500 to $700 price range. That’s better than the 9% who said they would be interested in buying the original iPhone in an April 2007 survey — after Steve Jobs had unveiled it, but before it had been released.

“The promising early interest illustrates the market opportunity for a Mac-based Tablet,” writes RBC analyst Mike Abramsky in a Wednesday morning note to clients.

Among the other findings in the survey:

• 25% of respondents bought a Mac laptop in the last 90 days, up from 18% in July
• 17% bought a Mac desktop in the last 90 days, up from 12% in April.
• 30% of 4,200 respondents in a separate survey already had an iPhone, up from 20% in June

It should be noted that respondents to ChangeWave surveys tend to be early adopters. They are drawn, according the ChangeWave Alliance website, from “a worldwide group of 20,000 highly qualified business, technology, and medical professionals … who spend their everyday lives working on the frontline of technological change.”

Given what he calls the “surging Mac and iPhone sales momentum,” Abramsky has raised his Apple price target to $250. Shares opened Wednesday at $185.35 and were up 3.7 points (2%) in mid-afternoon trading.