Does anybody know if it is necessary to make a reservation in Jerusalem for Easter? I mean, is it very crazy to make a reservation now ... with such and advance... ? Or it is convenient to do so? I cannot imagine how many people goes to this place in Easter...

It certainly is necessary - but not because of Easter itself.
Easter Sunday and Monday (March 31 and April 1, 2013) fall on the last 2 days of the Jewish Passover holiday - peak holiday season, when accommodation is scarce and expensive.
If this will be your first visit to Israel, I strongly recommend that you avoid touring during the Passover/Easter season. That period you'll find the largest crowds, heaviest traffic and highest hotel prices all year. And there will be plenty of Passover issues that could interfere with your touring.
Thank you, unfortunately -or not- we will arrive Israel on March 22nd, till April 6th. Although the crowds, I think once i life I would like to be in Eastern in Jerusalem, so I will prepare myself for the negative isues. We are planning the first two days in Tel Aviv, then from March 24 to March 30 in Jerusalem and then we will go to Amman, does avoiding the last two days in Jersualem. I hope it works!
Thank you for your comments!
B
You will be spending the entire PASSOVER holiday in Israel. This is the most expensive time of year in our hotels. If you haven't yet booked your hotel, do so immediately in order to avoid disappointment. Most Jerusalem hotels will be fully booked for the duration of that 7-day festival.