Welcome lenivix! There is a lot of useful information in these forums so be sure to spend some time looking around and using the search function.
First of all, are you looking exclusively at physics PhD programs, masters programs, or programs in other disciplines like biophysics? You say that most places list the PGRE as not required, but that is certainly not the case for physics PhD programs which overwhelmingly require the PGRE. The expectations and requirements change a lot from program to program, and your post is not explicit as to where you intend to apply.
Now, some answers.
>>Firstly, is it common for students to apply to graduate programs in their final (4th) year of their undergraduate degree?
Yes. Applications are typically due from mid December to mid January and the majority of applicants are in the middle of their final year of their undergraduate degree. The admissions decisions are based on your degree progress up until that point. Sometime after you graduate you are required to provide your new institution a copy of your completed transcript that shows that you actually acquired your degree, but when you are admitted it is just assumed that you will complete your degree soon. Some students do take a year off before applying though.
So basically, you can bomb your final semester and as long as you still have above a 3.0 and you still graduate, it won't affect anything (except fellowship applications and perhaps your future job prospects).
>>I was also wondering exactly how I should be converting my current grades into GPA...
Some schools provide an algorithm for doing this, others don't have you convert it and rely on your letter writers to provide insight into how well you did. Basically this varies from school to school, but there are also some generic conversion algorithms available. Regardless, be sure to have your letter writers mention what the class average is and how you rank in your class to give a better perspective of your performance for people unfamiliar with your system.
If 70% is the greatest rank one can achieve I'd imagine that's an A, but I'm sure some international applicants that have already had to deal with this system can provide a better answer to you here.
>>Also, generally speaking, do universities place more importance on degree results, or GRE?
This depends on the institution, but I think most schools are looking for you to demonstrate a certain level of competence on the PGRE. Once you achieve that minimum score, they look to the rest of your application to decide. Good grades are expected, and your letters of recommendation and research experience can have a very large impact. They may be the most important factors since GPA is hard to interpret and the efficacy of the PGRE is debated.
>> Also, when taking any of the GRE exams, must you list the universities you wish to send your results to before you take the test, or can you decide afterwards?
You can find this information on the ETS website. You are allowed some number (3 or 4) freebies which you list when you register to take the test. You can and will send your scores to more universities for a fee, and that is decided after you take the test. If you send your scores to a university and later decide not to apply there, the only consequence is that you lost one of your freebies.
For general advice, get as much research experience as you can. Everyone has good grades and decent test scores. Your research experience and the letters your research advisers write you are what make you stand out as an applicant.
For choosing schools, first go through the profiles threads to see what tier of school you should be aiming at. You want to apply to some schools at your level, some "reach" schools that are more difficult to get into, and some "safety" schools. If you know what you're interested in, then you need to start looking at the school's websites and finding research groups that look really interesting to you. If you don't know what you're interested in, look for larger programs with lots of different research opportunities.