When Medicaid pays for the nursing home care of one spouse, Medicaid rules allow the "community spouse" -- the one who still lives at home -- to keep a certain
amount of assets and income (including Social Security benefits). Your mother will be able to keep a total of between about $1,700 and $2,600 per month (in 2008); the exact amount varies from state to state. You'll have to check with the Medicaid office of the state where your parents live to find out the exact income limit there. This amount is the total monthly amount your mother will be able to keep, out of her Social Security benefits and his benefits, plus any other income, combined.
There are also Medicaid limits on the amount of assets -- Medicaid calls them "resources" -- your mother will be allowed to keep. Most importantly, she can keep her house (as long as she actually lives in it) if your parents own it, household goods, and a car; these won't be counted as assets to determine your father's Medicaid eligibility. If she moves out or sells the house, it would have to be used to pay for your father's nursing home costs. She can also keep between about $21,000 and $104,000 in other assets; again, the exact amount depends on the state where your parents live.
One more thing about your parents' house. Medicaid can eventually recover from the value of the house the amount it spends on your father's care. This Medicaid "recovery" out of the value of the house can only happen after your mother no longer lives there. But Medicaid may begin the process by putting a lien on the property while your mother is still there.