Geographic location of servers/host

[chantaspell]

Hi...I've been reading a lot about the importance of the geographical location of hosts and have a few questions...

The Background
I've had a website now for about 4 years. It is an information site about health which is relevant worldwide equally. I have google adsense on the site and it brings in a small income. Most of my visitors come from the UK and the site ranks well for its keywords in google.co.uk but the rankings in US google are not so good or non-existant.

The site uses the .com TLD. recently I used google webmaster to tell google that my site was of US interest, but as it is hosted in the uk maybe it will never be picked up by US google as well as it is over here. I made this change on google webmaster about a week ago,with no noticeable change in traffic location (still mostly UK)

The Questions
My 1st question is should i move it to a US host to take advantage of the bigger market, get more visitors and more profit from adsense?

My 2nd question is how long after moving will it take for google to notice the change and start showing up on US serps?

And 3rd and final question, does the google webmaster geographical location setting matter, and if so how long does it take google to update after u change something?

Many thanks for any help or ideas!