I would say that the ratio is about equal but unless you are prominent in either of your careers before you switch, people aren't aware of it. There are a LOT of people - especially back in the day. The norm now is women, but before, lots of men sang.
Did you know Eddie Murphy made a record? Oh yes, he did. :-D Others back then, too.
Singing to Acting:
Madonna, Britney Spears, Spice Girls, Jennifer Love Hewitt, another teeny bopper actress (can't remember) and I am quite sure we can expect to see Christina Aguilera make something soon. Mariah Carey. Toni Braxton. That Beyonce (or however it's spelled). Countless rappers. Ice T (cheese city), and Ice Cube (skill, but sometimes picks the wrong thing). Janet Jackson (Good Times, Poetic Justice). Michael Jackon (The Wiz). Diana Ross (she short of balanced the two - made a fascinating, odd film called Mahogany that freaked me out on TV the other day).
Acting to singing is actually quite common but this is normally when someone with acting tendencies also loves to sing so they actually meld the two sooner - into Broadway or off-Broadway. Or that's how they get in - they start by singing then get taken seriously. Just saw a show on Marilu Henner getting into the biz via this. Stockard Channing. Both started in Grease, right along with John Travolta. Our classic Bette Midler, who is grand and good at both. The infamous Barbara Streisand - come on! It's a long historic tradition. Fred Astaire, Ginger Rogers. Even our darling Audrey Hepburn in My Fair Lady, and Funny Face. All them yummy actors with humbling voices - Bing Crosby, Frank Sinatra, Dean Martin (oh god, his voice is velvet). Ah, velvet reminds me of Mr. Elvis Presley.
Oh yes, very common trend. And then people we become aware of through film who surprise us with their talent for singing, but they've done it a good while as part of their career - Jason Alexander, doesn't he? And the marvelous Nathan Lane. HE is purely born of the meld of comedy, music, and acting. I heard Kevin Spacey did well in the Iceman Cometh, does that involve singing? I heard that was on Broadway, too. Lots of actors go to Broadway, loving the stage, and of course, if they sing. And they leave Broadway to go to film. Lucy Lawless though, I dunno about her singing debut on Xena, that was just plain silly. I heard they recently did that with Buffy the Vamp Slayer. They should just let that go. Then there was that Woodly Allen film where they went musical. Spike Lee, wasn't School Daze a musical? Featuring Tisha Campbell, and others.
And of course our most recent development, Moulin Rouge. Nicole Kidman tooling around with Mr. Ewan Macgregor!
Musicals are a blessed artform - sadly, not all are talented enough to perform in them well.
Then there are those who want to go the complete switch, not musicals but a complete path. So, Jennifer Lopez, who began in music (dancing), got to flash the pretty lashes and pout for film and then show off those dance skills legitimately. There is no way for her to do so purely as a dancer and have that support her lifestyle. No one's gonna buy Lopez as just a dancer. Being what she is she can't just go dance in someone else's music video or Vegas revue - she's the star now and so, must take it over. She likes to sing and can carry a tune - natural choice (though overexposed and ill-advised) is to go into the music business.
Obviously Madonna's not done so well in the acting arena - I actually did not find her turn in Evita brilliant at all because she did not do much and she wasn't required to do much but dress well and look hopeful, sexual, determined, then ill. These are not hard things to do.
We're always gonna see a lot of this. It's an age old thing. Maybe it's the ancient link to singing in dramatic performances in ampitheatres in roman times. Or singing ballads by the fire that tell complex stories of life and love. The japanese have their Kabuke theater, in Indonesia they have their puppet theater (don't know the name), and in older cultures there's plenty of dress up and acting out of traditional stories - with plenty of singing. It' s in the blood.
Can't do one without trying the other - if you have the skill (or think you do, or wish you did).