there's NO reason to buy a G5 over an MX518. don't buy into the marketing hype. save your money.
seriously, no reason at all... even if you are a "hardcore" gamer.
the Death Adder is good... but you might as well buy an IE 3.0 which is much cheaper and performs just as well for all practical purposes. the shape is the same. costs around 1.5k
in fact, for the games you plan on playing... any cheap mouse that won't skip when you move it too fast will do.
remember this: there's a reason why even the most "hardcore" gamers (guys who are actually winning international tournaments for competitive games i.e professional gamers) are still using 10 dollar mice, although they're sponsored by and endorse companies like Razer.