I wanted to respond to this the other day but didn't have time until now.
I think what worries me the most is how leveraged you can become under your scenario.
For better, for worse, I think the magic number for putting money down on a house should be 20%.
So, if you want a house for 750K, you should put down 150K. If your mortgage is 3K/month. . .and you make 10K/month. . .25% of your income isn't bad, just about right.
I would liquidate your current house rather than taking a rental loss on it. You'll have very little capital gains tax and I do think a discussion with an accountant is order on that - rolling over your equity in another house seems to make sense.
Finally, where to invest your 400K.
Given your high tax bracket, I'd look to municipal bonds to stash your money, especially since you are going to inavoidably owe tax on your capital gains in 2008. So, you paid tax on that money once. Stuff it in a tax-free shelter. You can buy insured muni bonds, so you don't lose prinicipal and make 3-4% interest tax-free if you buy them in the State of California where you live. This effectively becomes a 5.5% interest rate for what I imagine is a high tax bracket for you and more importantly, it's risk-free. (well, it's principal-loss risk-free to be more specific, lest JimOhio jump on me about inflation-risk

)
You "ladder" them so when you need the money, you can get at it.
So 400K/year @ 3% = $12,000/year tax-free income to either a) reinvest or b) use for low spots in your income/safety net (that represents 4 months of your mortgage payments).
The next step is to set up a retirement account for self-employed (SEP-IRA, Keough) and start stuffing money in that earmarked for retirement.
You then have the 400K earning money tax-free and available at all times should you lose your business and you have retirement money growing in more aggressive investments (stocks+/- commodities).
My scenario is a bit conservative but I am a business owner like you. There aren't many here at savingadvice and it's different for us.
You take your risk to go out and secure money vs. just getting a paycheck. So. . .when you finally "win your money in the market", it pays to lean a bit conservative. This places a lot of your portfolio in debt sector bonds but as you add to your retirement accounts that will soon change/flip.
Good luck.