Indeed with martial arts (I practise ninjitsu) you shout a so-called ki-ai (just like you I don't know how you're supposed to spell it). This burst is used for two reasons. Reason one is that it intimidtaes your opponent. If you got a powerful ki-ai it will show great confidence in yourself. If somebody would give a lowsy ki-ai then not a single person would feel intimidated by the shout.
The second advantage is that it increaes the person's power at that moment. Since you breath out when you scream you will increase your power. Try it out yourself; kicks with your breath hold inside your body are less powerful then kicks when you breath out (especially when they are combined with a ki-ai).

When comparing this with the DB saga it indeed shows that the last HA of the Kamehameha is a lot longer than the former parts of the attack. Apparently Goku increases the attack by screaming the name of the attack. I'm just guessing this, but it sounds reasonable to me.

About your question Animaniac...for some attacks there will be a longer power-up needed then for others. For an example a kamehameha is a lot faster than a spirit bomb.

I'm looking forward to receiving the Neko comics ^^

Also on a little note..I have seen the last DBGT episode now..yay..haha. Besides that I started downloading more DB again lately..so I'm at episode 47 now..haven't really progressed a lot I guess..hehe.

Peace out