I've just learned this first hand the hard way. Here's the basic run down. There are two type of stainless wire rope that have been widely used. They are a 304 alloy and a 316 alloy. The 304 alloy is stronger than the 316 alloy, but the 316 alloy is more corrosion resistant. For example, 7/32 wire in 304 has a tensile strength of about 6300 lbs. 7/32 wire in 316 has a tensile strength of about 5250 lbs - about 1000 lbs less. If you are replacing 304 wire with 316 wire, you need to go up one size to account for the difference in tensile strength. Finally, most older rigging e.g. original equipment on Ericsons, will be 304.
Do be aware that there are points where pin size change as you go up in wire size. In my case of replaceing 7/32 with 1/4, I didn't have much of an issue.
A final thought, if you are doing all your standing rigging, spend the extra couple hundred bucks to replace all mast tangs, toggles and bolts that attatch the stuff. These pieces wear out also. In my case, my mast was made by LeFiell, and I was able to have all the stuff on my doorstep for less than $250.
-David
Independence 31
Emerald