Re: Just got a VS3
J&J will fit.
As far as the tank, the 4500psi and 3000psi is merely the pressure the tank can hold. Obviously, the 4500psi tank holds more air, and will last you longer. They're also lighter normally, as the small capacity (48ci, 45ci, 72ci) tanks are normally aluminum, while the 4500psi tanks are always carbon fiber wrapped. Some 3000psi tanks (68ci, 92ci, 114ci) are carbon fiber wrapped, but are the same price or just a liiiiiiiiiiittle cheaper than a 4500psi tank. I'd personally go for a 68/45, fits almost every person perfectly for the proper form and is the industry standard to base efficiency on (1500 shots per tank, etc).
The 800-900psi that you mentioned being advertised is the output (HP, High Pressure in this case), which is the same output as a CO2 tank. LP (low pressure) tanks put put 400-500psi.