"M20B257" is a nickname for using the M20B27 block or crank along with an M20B23 or M20B25 head and electronics. This provides a 2.7 litre engine with the much improved flow dynamics and valvetrain components of the M20B23/25.
Other names it is known as: M20B25+27, M20 "stroker" (as you increase the stroke over the stock M20B25), M20 "Frankenstein" (as you combine different parts to form one whole, much the same as Frankenstein did in creating his monster).
This question requires a rather intensive answer that we hope to cover completely on this wiki soon. Until then, we recommend reading over the community information available on various forums and wikis around the internet. The parts required vary depending upon your vehicle year and your goals, and there are quite a few options to choose between. We believe it would be most beneficial to read through others’ experiences to determine what you desire for yourself. A great thread on this subject is here: http://www.r3vlimited.com/board/showthread.php?t=100939