This thread is about 12 months old now . . . but in all this time, other than saying the best center channel to use is the same type of speaker that you are using for the fronts and rears, no one has mentioned the following (simple) solution to your original question, which was, "what center channel should I use with the Monoprice bookshelf speakers?"
I would recommend, like others have already mentioned, since you are only trying to build a modest (inexpensive) surround sound set-up, and Monoprice speakers are so inexpensive, that you purchase 3 sets of bookshelf speakers (6 speakers total), and a monoprice sub. That will give you two fronts, two rears, a sub, and you can use one of the bookshelf speakers for the center (a 5.1 set up) by simply laying it on it's side, either under or over the TV.
But what to do with the extra speaker . . . I'll get to that in a second . . . please read on.
The monoprice bookshelf speakers have enough frequency range to do modestly to reasonable well as a center speaker (60 Hz to 20 kHz or 60 Hz to 20 kHz depending on which monoprice bookshelf speakers you purchase). And by adding the sub, well, compared to the old stereo system you've been using, you'll feel and think you're at an iMax Theater with full THX sound (but you won't be, sorry).
IMHO, this set up will work nicely, especially compared to the stereo sound you've been accustomed to hearing for the last 10 plus years.
But you wonder what to do with the left over speaker? Simple, put the remaining speaker in another small room, like a restroom, kitchen, hallway, etc., get a monoprice speaker selector switch (either PID #9995, 8229, or 8230), and connect the center channel to it. Then from the selector switch, connect to the center speaker, and the extra speaker in the hallway.
By doing this, you'll have the center channel sound, which carries all the spoken audio of a movie with minor background sounds, and all the spoken audio of music with the music mixed in, in the hallway or other small room (I have mine in a small washroom/bathroom combo). If you are like me, I get up sometime and run into the restroom but don't want to stop the movie, tv show, or music. But by having installed the lone speaker, and setting it up as a zone 2 with your speaker selector switch, you can hear it in the other room and not miss a beat of your favorite song, or television episode.
You can do all this for about $223 plus shipping from Monoprice (3 sets of speakers @ $30 a pair [$90), sub at $106, speaker selector $27).
This is an inexpensive solution, especially for speakers.
Having said all this . . . are there better speakers out there? Of course there are! But bang for buck, I think you'll be extremely satisfied with the set up I'm recommending (compared to what you've been listening to for the last 10 years).
But so you know, Monoprice does a have a 5.1 speaker package, too. However, take a look at the specs on that offering. The speakers frequencies are NOT as good as purchasing the individual monoprice bookshelf speakers and making your own 5.1 set up, and all for nearly, if not less, then the Premium 5.1 speaker package they are offering.
I have personally spoken to the Monoprice Tech department about this, and they have confirmed I am correct. The individual speakers DO have better specs, and do make a better 5.1 or 6.1 set up compared to the Premium 5.1 Speaker Package.
FYI . . . I am only suggesting any of this because you are looking for a simple inexpensive upgrade from an old stereo. if you have the money, or can save up, there are MANY companies that make wonderful speakers and audio products that would blow away anything I have suggested. But if you are looking to only upgrade from the old stereo in your house, you will be pleasantly pleased with the solution and sound of the products I have suggested above.
I should also say . . . I do NOT work for Monoprice . . . but I do have three audio set ups in my home, with 5 zones set up throughout the house. One AV/speakers set up is inexpensive, one is moderate, the last cost me more than any sensible man should spend on audio equipment for the size of my rooms. So I speak from hands on experience . . . if you have the cash, spend it on quality av equipment and speakers, but if you don't have the cash . . . you'll be pleased with the Monoprice speakers, really, you will.
On a personal note I'd like to say, be careful, once you get the speaker sickness you'll never be the same!
Quick story about getting the speaker sickness . . . . my wife, who thinks listening to music from the tiny speakers on her cell phone sounds good, thought I was nuts when I started to install all the audio stuff throughout the house. Then a few months went by, and she went to visit a friend. Her friend has a Home Theater in a Box set up. When she got home she said to me, "We watched a movie, and I couldn't believe how poor the sound quality was." I laughed so hard that I nearly fell off the chair . . . she looked at me and said, "What's so funny?"
Oh . . . and now she gets upset when the sound isn't just so when we watch a movie together. She's got the audio/speaker sickness.
Watch out, once you start on the road of speaker upgrades, you WILL be susceptible to the speaker sickness!