g400 is basically a mx518 with a different colour and more DPI.. don't get it if you're bored with the mx518. Instead invest 300-500 rs more and get a Razer Deathadder: Black Edition.That's the best you can get.
Btw don't even think about steelseries kinzu (rs1200) it has the shittiest sensor ever and even some local company mice are better than it :/.
Or maybe you can get a Microsoft Intellimouse 3.0.Excellent mouse.Good shape. Only issue would be if you play at very low sensitivity. Otherwise all pro players in CS and Quake used to use either a 3.0 or WMO (same sensors for both of em';one of the best sensors ever)
I'm in the same situation as you, have mx518 and bored of it. I've done quite a bit of research now and im buying the DeathAdder: Black Edition.