im building a new rig, and need to know if theres really any noticeable, real-world performance difference between a 500gb drive with 16mb cache or a 500gb drive with 32mb cache. both would be running on Sata III 6gb/s
Of all the factors that affect a drive's performance, cache is by far the least important. The most important factors are the spin rate (RPM) of the drive and the platter density. More spins per second = more data per second, and higher platter density means more data per spin = more data per second.
Cache is important, but once you get beyond a certain amount it doesn't really give you any better performance. Since cache memory is cheap, all the manufacturers include enough of it to optimize the performance of the drive. As a result, differences between one drive and another don't really make a noticeable real-world difference.
In fact, the most common reason for different cache sizes is because the platter density is different and so the drive needs more cache to hold "x" tracks worth of data. In this scenario the drive with more cache will perform better, but it's because of the higher platter density, not because of the extra cache.