Well, if you want something practically identical, then the G400 is what you're looking for, as it's really the current version of the MX518.
I prefer the G500 over the MX518/G400 in general, but depending on exactly what you're looking for, you might be better off sticking with the optical mice. As far as I know, all laser mice have at least a small amount of hardware acceleration, which really isn't ideal for serious gaming, but you do get used to it. The G500 (and G700, I believe) also has a scrollwheel that's toggleable between a normal-ish mode and a unstepped flywheel mode, which is pretty cool. However, it does make it possible to accidentally scroll while middle-clicking, even in normal scrollwheel mode. The G500 has extra buttons and adjustable weighting and whatnot as well.