1) what are the differences?
Well, first, you have to be clear when you mention a "new version of the King James Version". Here are the primary versions available (but there are others). KJV = King James Version
a) "Standard Text" - this is essentially the 1769 Oxford Revision and the basis for nearly all KJV bible editions in print.
b) "New King James Version" (NKJV) - this is a new translation of the source texts used for the KJV. The KJV is used as a reference, so the wording is very similar. Unlike the original KJV, the NKJV omits the Apocrypha.
NKJV web site
http://www.thomasnelson.com/consumer/dept.asp?dept_id=19700&TopLevel_id=190000
NKJV video
http://www.thomasnelson.com/consumer/downloads/nkjv_medium.mov
c) New Cambridge Paragraph Bible (NCPB) - this is a scholarly attempt to reproduce the original translation of the KJV. Even though spelling is modernized, it is probably the closest edition to the original translation ever in print.
More info:
http://www.bible-reviews.com/selector_kjv.html
2) I learned in history it was the oldest translation of the Bible in english
This is not accurate. There are at least 6 complete English versions of the bible that pre-date the KJV. The oldest is the Wycliffe bible of 1382. The KJV was first printed in 1611. Other complete English translations that pre-date the KJV include the Matthew Bible, Coverdale Bible, Bishops' Bible, Geneva Bible, Great Bible and Douay-Rheims Bible.
3) Full bible text online
KJV (Standard Text)
http://www.biblicalproportions.com/modules/ol_bible/
NKJV
http://www.biblegateway.com/versions/?action=getVersionInfo&vid=50
Jim