I find it hard to tell the difference as well, especially when gaming. In static screenshots with the aid of zooming in then its possible to see the difference, but if you're playing something like counterstrike or dod imo 4x or 16x its all the same, you're not gonna notice anything gobsmackingly obvious that you're "only" using 4xfsaa.
SP games on the other hand as much as the gfx can handle
. Even underpowered they take a hit higher than the GTX/Ultra.
. Sold for £107 on a famous auction site less than a year ago
)