The normal GRE is usually required by all graduate programs no matter what area they are (aka somebody applying for entry into a PhD French Literature program will have to take the general GRE just the same as somebody applying to a PhD program for Physics).
The Physics subject GRE is usually required when you apply for graduate level physics programs (sometimes the Math GRE is also required, but that is MUCH less often aka very rare that a program ask for both subject tests).
The general GRE tests your basic Quantitative, Verbal, and Writing abilities much like the SAT did back when you were in high school. The Subject tests basically cover ALL (or at least 90% or more) of the material you should have ideally encountered during your undergraduate education in that field.
This means the Physics GRE covers: mechanics, electromagnetic field theory, thermodynamics / statistical mechanics, optics, modern physics / relativity, quantum mechanics, electronics, and laboratory techniques.
The Math GRE covers mainly: calculus, linear algebra, differential equations, abstract algebra, and real analysis. It also contains a few questions from any of the following: complex analysis, PDE, topology, discrete mathematics, statistics, game theory, bifurcation analysis, vector analysis, differential geometry, etc... the large majority of the test (like 75% is those first topics I mentioned ... calc/algebra/analysis) and the rest are those less general areas of undergrad math that not every young mathematician would have been expected to be exposed to.
As far as test prep materials, I'd go with Kaplan or Princeton Review as your main resource. I always felt that McGraw-Hill, REA, Manhattan, and other test prep materials were not nearly as good as the Kaplan or PR if you only end up using one resource to prep. If you end up using multiple, then yeah any of those others used to supplement is totally fine, and I'm sure if you went with actual tutoring from real people, I'd imagine they're all about the same since they all have a wonderful screening process for their instructors.
Most programs are probably looking for scores in the top 90% (if not top 95% or higher) on the quantitative section of the general GRE and at least top 50% on the subject specific test ... the reason for this is, well think about it ... on the general test, you're competing with English Lit majors, Philosophy majors, Business majors, Art majors, History majors, etc... so if you, as an eventual Physics PhD candidate aren't scoring higher than 90% of these people on the quantitative reasoning section, it's not going to look very good. But when it comes to the subject specific one, it'll really depend on the programs you're applying to. The subject tests are being taken by all the same type of people ... the ones looking to apply to grad programs in that subject, so they all have a relatively similar background and yeah, if you can set yourself apart by being the top half, or top fifth (being at least a standard deviation above the average tester), then good for you.
You shouldn't stress the GREs TOO MUCH though, they are often more of a comparison rather than a pass/fail test when being looked at by admissions committees. Now that they changed the general GRE, there's not as much you can do to better your score in a short amount of time (like before you could cram 250 of the most common GRE vocab words in about a month and end up with a score 100 points higher out of 800 on your verbal) but now doing that may net you 1 point higher (out of 170). I'd concentrate mostly on the Quantitative section of the general GRE and mainly focus on time management when you're taking a few practice tests ... that will help out more than anything else.
Good luck, sorry this post got a bit out of hand with length and organization.
