2. The final version has been released to particular groups of Windows users and manufacturers. For the general public, it will be released in March or there about.
3. I remember reading articles (cannot source them right now), which says that it affects some security suites (i..e antivirus, firewall, antispam), so you should check the website of whichever security software you use before installing SP1.
5. Yes and No. When you first install SP1 it will be slower than before when it comes to launching programs. This was stated by Microsoft and others who have tested the speed of a SP1 system after an upgrade. Also the following article mentions that even Windows XP is faster than Vista SP1 in some scenarios
Microsoft: Windows XP faster than Vista SP1 in some cases