Buying house over $500,000... Do I have to pay tax on it to the USA/IRS?

DH is a Brit and I are about to buy a house and I thought there was a law about people who buy houses that are more than $500,000 and that you have to pay taxes on it to the USA. I currently am not on the mortgage as DH bought the house before me and the house is less than $500,000 but the house we are about to buy is over $500,000. I do not have a home in the USA. Any advice?

Trying to decide if I should be on the mortgage for it as well.

Capital Gains Tax on the sale of your home is only due if the profit from the sale is over $500,000 (married filing jointly) or $250,000 (married filing separately). Therefore if you buy a home for $400,000, and sell it for $640,000, you would owe no tax in either case. There are also reductions that can be made to the sales price to arrive at the profit. You can learn more at:

http://www.irs.gov/pub/irs-pdf/p523.pdf
Selling your home

Caution, be aware of exchange rate rules at the time of purchase and time of sale.
EDIT: There are additional rules about periods of time you actually live in the home, etc. (Ownership and use tests). They are covered at the above link.

How are you filing for tax? If you are "married filing separately" and you husband has no US tax liability then the sale of your husband's house has no US tax implications.