Looks like you have good help with people who can fill you in on PhDs!
As for universities, Yale is what is called an "Ivy League," school here; it's one of the top-rated universities in the country.
However, if you plan to work in the States, where you get your degree from isn't always the most important thing considered by employers (I don't know about the UK). This is sometimes the case, however it may not be in your case so you should REALLY look into it. Yale is incredibly expensive and very demanding. If your planned career field won't be one in which potential employers focus a lot on the name/reputation of the university you graduated from, maybe consider something other than Yale... then you'll have time to explore the area a bit more :)
My PhD program leads to a career in which employers won't focus mainly on the university name I get my PhD from, so I chose a university with a good reputation but won't knock me out with work.
Keep in mind, you'll likely end up taking the SAME classes no matter where you go.
Here are the "top 25 phD geology colleges" in the USA:http://geology.about.com/cs/colleges/a/aatop25phd.htm
There are a few Ivy Leagues in there, so just take the time to decide if you really want to apply to one of those. It's a rigorous application process and they will likely be incredibly demanding.
As for the idea of getting a PhD, go for it! It may be quite a bit of studying to get it, but it'll be worth it in the end.
Check out this bar graph for the average differences in annual salaries based on education levels/degrees:http://www.ohe.state.mn.us/tPg.cfm?pageID=948