NEVER USE BUILT-IN FLASH!
You should use a pair of studio strobes for this job and bounce them into umbrellas.
Price-wise, you can't go wrong with a pair of White Lightning 5000 strobes at $80.00 each. These models are like the Energizer Bunny "They keep going and going!" I have used a pair for 20 years and they were purchased used.
WHITE LIGHTNING 5,000 STUDIO STROBE FLASH MONOLIGHT - eBay (item 350267532000 end time Nov-19-09 09:32:17 PST)
Two stands and a pair of umbrellas can also be cheap on eBay.