There is no hard and fast rule on this one.  Some older cats are neutered
and they do stop, some do not.  It is recommended that you neuter them
before they become sexually mature and even know about it so that they don't
do it.

It will stop them from spraying their potent scent all over, as with out the
testies, it won't be as potent, and they'll stop smelling like smelly
underarm.. but they might still spray, and they might still wander.

Good luck!

Well, you didn't state the age of your boyz, but generally if you can have
them done before they reach sexual maturity (at around 7 months old) you
should be OK, and they will not have developed that roaming instinct that
much. They might not even spray at all. Saying that, like the other poster
said, even if they do spray, the smell will be non existent, just tested it
on my Charlie, the former stray, who lost his crown jewels mainly for that
reason  as once he moved himself in he sprayed the flat all over :)

Should your boys be older, they will still do what they are doing now, but
no they will not bug girls in a  sexual sense, as there is no sexual urge,
nor the sperm factory anymore. They will also wonder less, and fight less,
but as everywhere, there are exception to the rules. My neighbor's Barney is
~10 yrs old, missing most teeth, long neutered, but still fighting like mad,
and a famous local cat bully that scares other (unuttered) cats up the tree
! :)

Not nececerily, in UK, vets do not perform the op before 6 months old.

Gee

My cat was neutered last summer (at around five months old) and while it
doesn't spray or anything, it does still do the motion of "humping" on
some pillows and such.  It was kind of disturbing, but at least he
doesn't spray or anything.  Kind of odd though.  ;)

Eric