Hello mate. Welcome to the forum.
I, too, am 21 years old and just recently purchased my Z.
I've insured it with Admiral on their 'Multi-Car' scheme. That covers for every car that is on your drive, so if you're still living with your olds, then this would be the one to go for.
Basically, I have 12 months on my '08 350z from Aug, then when my Mum's car is up for renewal (Jan '10), and my Dad's (Apr '10), their cars are fully covered from these dates up until mine is then up for renewal itself.
So, for 24 months worth of car cover I got it for £1054! Most other quotes did not even come close to this, as I'm sure you have already discovered!
Hope you find the right car, and the insurance isn't too much to make you flee to the toilet.
Good luck.