http://www.defenselink.mil/mil...2007_apr_basicpay.html
Breakdown of all military pay grades.
Flag officers (i.e. Admirals, Generals) are n the O-7 through O-10 ranges (one higher grade is for time of war only, usually. Last used in WW2 if I recall).
For the US Army, a 1-star General (O-7) is called a Brigadier General, a 2-star is a Major General, 3-star is a Lieutenant General, and a 4-star is a General. Navy equivalents are Rear Admiral (Lower Half), Rear Admiral, Vice Admiral, Admiral. US Marine Corps' and Air Force's titles mirror the Army's, but are abbreviated differently for ease of exposition.
There are a limited number of billets available at each level, and all require Senate approval after nomination from the Secretary of Defense.
So for a full, 4-star general officer (i.e. full Admiral or General), with 32 years in the service + a family of four, stationed in Washington DC, a realistic basic pay allotment would be along the lines of $168k/yr, plus housing allowance of $30-35k/yr, + food allowance of ~$3k...so in the neighborhood of $200k/yr is pretty normal.
Once you add in some other monetary perks like hazard pay for those in dangerous areas, those serving in NATO or other foreign billets, uniform reimbursement, and so on, you could reasonably have a salary of ~$300k for a 4-star officer.
Now keep in mind the number of 4-star officers is limited, by statute, to 32 total (for all services, broken down by "need"), but is often exceeded because certain positions require a 4-star officer and don't count against the 32-person cap, or by national emergency like the post-911 period. Currently around 45 4-stars if my last count is right.
A common pattern for retiring flag officers of all levels is to go work in the private sector afterwards. In my experience, they usually make at least double, often quite a bit more, of what they made in the service. Some often return to government service as civilians in various posts, often of higher salaries.
Federal service, on average, make 23% less salary than an equivalent private sector position. Some government jobs obviously have no equivalents, and vice versa. This is according to a November 2007 newsletter for federal employees. The pay-gap has increased over the years from around 15% in the early 1990s. However as a civil servant, particularly those who are permanent appointees, the job security is unmatched (can't be fired unless you do something BAD) and the benefits are good to very-good, particularly for uniformed and civilians who serve 30+ years get VERY good retirement. Civilians pay is changing quite a bit now, particularly with most the Defense Department shifting from the GS scale to one called NSPS...a lot of questions about its short and long-term impacts still.