Look. You're 40. It's time to sit down and think for a while. No, really. Just sit and think.
Think about where you've been, and where your career has been. Think about where you want your career to go.
Factor in the fact that you probably have 20 more years of work ahead of you, if you're lucky.
Think about all the things you hate about the current scope of your work, and think about whether you still want to be doing them when you're 50.
Think about the things you can do if you hire two young buck programmers, teach them right, and have them do your programming for you.
Think about what people have said about what you excel at, and if it's different from what you've been doing, think about going that way.
Think about going into management.
IT (I'm using IT as the whole sphere of computer related degrees) + MBA = CIO position.
Think about that. Even if you don't make it to the CIO, and you may not, what you're buying it looking at problems from the 50,000 foot view and choosing the direction of a company.
If you get a master's degree in BioInformatics, you'd better focus on doing something no one else is doing, preferably something forward-looking. By the time you get done with school and what not, it's probably not going to be as forward looking as you thought.
Computer Science could be easy, it could be tough. You've got programming experience, that's great. You're probably reluctant to pull all-nighters though, and there's some 18 year old kid who will. And what is it really going to get you?
Here's what you do.
Look around the world. (Yes, the world.)
Find the people who are a few years older than you doing work that you think you'd want to do.
Shoot them an email. No, seriously, shoot them an email. Get over that fear of "Oh, they're too busy and don't have time for my little measly email." Politely lay out your case, and wait for their advice. It might take them up to 3 weeks to reply. You might have to email them a few times (no more than once every two weeks).
Find out how they got there. Master's degrees? Fellowships? Luck?
Follow in their footsteps as much as you can.
Just remember, you're 40. If you were to quit work and go to school full-time, you'll be 42+ by the time you graduate. In two year's time, some jobs you want will no longer exist. They will have been discovered to be dead ends (see: Ruby on Rails in large applications) The industry will have moved on. You'll have to be above that on some level. Figure out whether going to school is going to pay off. You can finish undergrad in 2.666 years if you set your mind to it. A master's will take two years no matter how you slice it.
Here's a hint: A Master's degree is not as much about the education as the opportunities you will be provided as a side effect of the school you go to and the people you meet there.
Therefore: Go to a good, real, school. Discard the University of Phoenixes of the world right off the bat. Apply somewhere there are people that might be smarter than you. Many companies use specific Master's programs as feeder schools. (Stanford -> Apple + Google, Cornell -> IBM).
Remember, you're already 40. Going back to school is going to be weird.
But it will probably be rewarding.