If it is shaking at high speed and gets worse with braking, it sounds like you have either a very worn wheel bearing, or very badtie rod.

Take your car to a front end shop quickly.

 

I had the similar experience. Turned out one of the wheels were slightly bent. Even the road force balancing didn't fix it.
I had the stock wheel set in the garage, which worked perfect. If you haven't had any roadforce balancing already, try that first.

 

are the wheels centered and do you have spigot rings on your alloys
http://images.google.co.uk/images?hl...ot+rings&gbv=2

do you have rotational tyres on there and are they the right way?