Mary-Kate Olsen and boyfriend Nate Lowman are about to face off over a staggering $900 million prenuptial agreement before their wedding, say sources.

The 23-year-old star, who has amassed an astounding billion-dollar fortune with her twin sister Ashley, thinks artist Nate is the man she wants to spend the rest of her life with – and her legal team will make sure her massive bank account is protected BEFORE the couple walks down the aisle, say pals.

Mary-Kate began dating the handsome 30-year-old in early 2008 after they were introduced by friends. Although she was dating other men at the time, Mary-Kate quickly realized there was something special between her and Nate, and decided she wanted to see him exclusively, says a close source.

"Since then, their love has grown to the point where it’s time to take the next step in their relationship. Because many millions of dollars are at stake, there’s no question that Mary-Kate’s legal advisers will make sure a prenup is ironed out, and Nate’s signature is on the dotted line," said the source.

Mary-Kate is determined to be a mother by the time she’s 25, say pals – and she believes Nate will make a great father. Mary-Kate has all the nurturing qualities it takes to be a great mom.

"There were plenty of guys who came before him who were wealthier," said the source. "But she knows, because of her tremendous wealth, her son or daughter will never want for money. She’s at a place in her life where she wants to move on to what really counts – getting married and having children."

While Mary-Kate’s rep insists "there’s absolutely no truth to any of this," sources tell The ENQUIRER that Nate can definitely see himself married to Mary-Kate, and expects the prenup will be hammered out in the near future. He’s fully aware that Mary-Kate has a fortune to protect and understands why an agreement is necessary.

While some of the artist’s friends believe Nate should agree to whatever Mary-Kate’s legal team draws up, "others think he shouldn’t sign it too quickly – that if they do marry and things go south and it ends in divorce, he should be in a position to walk away with a big chunk of change," said a close pal of the artist.

"What’s a payout of even $50 million to someone who has an empire worth more than $900 million?"

Mary-Kate and Ashley are partners in the billion-dollar Dualstar Entertainment Group. Ashley will probably encourage a prenup to make sure there’s no threat to the business they worked so hard to build.

"Ashley is deeply invested in their success and loves Mary-Kate with all her heart. She would never let anyone take advantage of her sister," said an Olsen family insider. "She knows a prenup is important to protect their vast fortune, and she certainly would look down on Nate if he contests it."