If you put $20,000 down on $30,000 note you would definitely need to go to a bank or credit union and your payments would really depend on how many years you financed over.
If you have any questions feel free to call or email any time.
Payments really can be varied depending on your financial circumstances and the loan program you are using. Your best bet is to contact a good lender and have them work up the numbers for you on a particular home. But since you don't get back here until September, I'd recommend that you contact a good lender NOW to find out how much you will qualify for, and then start looking at properties when you get here. Properties available now won't be available then and there will be new properties available then that don't show up now. I can put you in touch with a great lender that I work with all the time.
If you buy this year, you can take advantage of the $8000 tax credit so I would definitely recommend that you pursue your home purchase. $20K is a good down payment and Dallas area is one of the most economical places to live in the U.S. so I am confident that you will find many homes to choose from.
If I can help you with more info or a referral to my lender, please let me know!
Have a safe trip back to the U.S!