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There is no limit on the gift value - you may give any gift - but should consider tax consequences.
A recipient of the gift - does not need to claim it as income. Please see for reference IRS publication 525 page 33 - http://www.irs.gov/pub/irs-pdf/p525.pdf
The donor - if he/she is an US person - would be required to file gift tax return if the gift is more than $13,000 per person per year (for 2009). There will not be any gift taxes unless lifetime limit of $1,000,000 is reached.
There is an unlimited gift tax exclusion available to pay someone's medical or educational expenses.
In your situation - your parents need to file gift tax return.
As each gift your $50,000 - taxable gift is $50,000-$13,000=$37,000.
So after that gift - each still will have lifetime limit $1,000,000-$37,000=$963,000
Let me know if you need any help.