The strength of computers at chess is a bit of a complicated subject. Chess computers are really only very good at one thing (calculation), while the bulk of the program is there to cover up the weaknesses (everything else) as best as possible. When you see a human vs computer match the majority of the heavy hitting is really just the computer selecting moves from a database of human games, relying on human strategy, to carry it through hopefully to a winning position. However while all this is happening it's calculating and (somewhat badly) evaluating millions of positions, which means it doesn't make any tactical mistakes.
For some reason a computer playing from a database of pre-selected human games just doesn't sit as well for me as if the computer were actually finding the best moves through it's own calculation.
Also of note is that even with the massive database and relentless calculation, humans can beat computers at correspondence chess where the humans can spend enough time to calculate out everything just like the computer does. It's the time limit that makes their calculation so strong.
But to answer your question more directly, computers are rated somewhere around 3500 (although their rating has more to do with beating other computers), while Magnus Carlsen is rated 2870. However despite a 600 point rating difference, I'd expect he'd draw the majority of games against computers in a match.