Major Version Upgrade - 4.11 to 5.x

[Valen Jones]

I'm interested in upgrading from 4.11 to 5.x.  I currently track 4.x 
stable using cvsup, but I've never done a major version upgrade.

First, should I bother?  My hardware has dual pentium 1.13 processors 
with 1G ram (I'm considering maxing it out at 4).  I host a few domains 
on this machine and I have four jails configured on it which will have 
to be upgraded too.  I have users counting particularly on mail service 
not being down for too long.

Other than the obvious advice to start with a good backup, can anyone 
tell me:

1)  Will I gain a major benefit from upgrading
2)  Where should I look for instructions / advice on upgrading
3)  Also any general advice from personal experience.

Thanks.