Did tactics training improve your chess significantly?
As BlueRaja said, chesstempo is good, but you can train tactics @ FICS, chess.com, gameknot.com, chesscube.com...etc

Famous quote "1 problem a day keeps the patzer away"
Indeed when you start doing tactics regularly you immediately see results.
You tend to do a lot less blunders (which obviously takes you a rank up), remember some positions and tactics.

There's also some tactics training that I will describe as an answer to your other question.