The wireless MX700 keyboard+mouse combo I have right now works great for gaming. However, batteries add weight and weight makes precision shots a little harder to perform. Also, finding out that you forgot to charge your mouse the night before by having it start dying mid-fight will get you mad. If you do a lot of gaming, like myself, then you'll find that your mouse needs to be recharged nightly, instead of every three nights like the box says.
Definitely get a wired mouse for gaming. The MX518 is probably what I'll switch to when it's available in my area and my wireless combo will move to my htpc.