www.tirerack.com has a tire size calculator that is very good. It can be a bit tedious to compare multiple tire options, but you can build a chart with pen and paper.

I have a calculator that I made in Excel that calculates all of the pertinent dimensions and gives a spreadsheet of the results.

Basic Information
In a tire size of 235/65x17 -- factory base tire/wheel fitment -- the numbers, 235, 65, and 17 can be used to tell you the circumference of the tire. You can then change numbers and find the circumference of a different tire.

235 is the width of the tire, 65 is the aspect ratio or sidewall height as a percentage of the width, and 17 is the diameter. 235 and 65 result in numbers that are millimeters, and 17 is in inches.

Painful Details
For any practical purpose, if you have a tire width of 235 and want to keep it but also want a larger wheel (rim) then you have to reduce 65 by 5 for every inch of diameter change. For example, if you have a 235/65x17 and want to go to a 19, then you would need a 235/55x19 to have the same circumference tire.

This might not look very good, but it is a description that lays a foundation.

If you wanted to bump the 235 (width) to 315, then the aspect ratio would have to come down else the resulting tire would be to tall to fit the car. A 235/55x19 will have a sidewall height of 129.25mm. Dividing 129.25 by 315 gives us .41, or 40% (rounded). This tells us that you would want a 315/40x19. This should give a tire with a similar circumference as the factory base model tire.

You can use the tires on your car now as the basis of the calculations. Reduce the aspect ratio by 5 for every inch added in diameter of the new rim if your current aspect ratio is 55 and you are getting wheels that are 2 inches larger, then the new aspect ratio should be 45 to end up with the same circumference.

Multiply the current width by the new aspect ratio to find the sidewall height that is the target for the wheel.

Divide the target sidewall height by the desired new width to find the aspect ratio for the proposed width. You have to round the nearest 5%. (129.25 / 315 = 41, so the aspect ratio will be 40).

Because your X5 is all wheel drive, then you want the circumference of the front and rear tires to be as close as possible to identical. Identical is not likely to happen, but you need to be as close as you can get.

width x aspect ratio = sidewall height (in millimeters)
(sidewall height x 2) + diameter = outside diameter (in millimeters)
outside diameter / 25.4 = outside diameter in inches
outside diameter in inches x 3.14159 (pi) = circumference
5280 / circumference = revolutions per mile

You want the revolutions per mile of the front and rear tires to be as close as possible to the same. There is a variance that is allowed, my instinct is about 10 revolutions per mile difference, but I don't know for certain.

BMW offers a staggered fitment option in the tire packages of the X5M. If you looked up what the tires sizes are for the front and rear, you could determine the variance that the factory is okay with, then you would know if the tires you are looking at are within the variance.