The L-1 visa is a good one if you qualify for it (see
http://www.usembassy.org.uk/cons_new/vi ... work2.html for details).
If you do qualify, you should also ask about your company rules for sponsoring you for a green card if you decide you want to stay for good. The downside of an L-1 is that you have to leave the country if your employer decides to fire you, and you can't move companies on an L-1.
If you don't qualify, then you'll probably be looking at an H-visa.
I've only been in CA for a couple of months, but I'm already finding that US tax is a tad complicated. (For example, your State tax contribution is a deductible on your Federal tax). And it does depend very heavily on your personal circumstances. Here are the current taxation rates to give you an idea of your tax burden:
Federal -
http://www.irs.gov/formspubs/article/0, ... 56,00.html
State -
http://www.ftb.ca.gov/forms/05_forms/05_540tt.pdf
But tax isn't everything. Another big deduction is health insurance. And there's a few others as well. Your best way to get an idea of total salary deductions is to find someone in the US store you want to move to and ask them what their deductions are.
Remember as well that (except for housing in some areas) the cost of living is much cheaper in the US than in the UK, so don't simply translate your disposable income back to GBP to decide if you can live on your net salary. You need to work up a budget to include everything fom accommodation and motoring costs to grocery and electricity bills.
Cheers,
Stephen