With a proper cable, speed/negotiation settings and decent hardware a wired connection will ALWAYS - ALWAYS out perform a wireless connection from every angle except ease of setup.
From a LAN point of view, 802.11N can theoretically outperform fast ethernet. In reality that is simply not the case. Especially since these days gigabit ethernet is becoming an easy reality.
However, for the purpose of Internet gaming that is fairly unimportant since most people have 5-25Mbps Internet connections. So the SPEED of wireless won't be the major issue. Wireless issues..
-Consistency of speed
-Initially getting speed that maximizes your Internet connection
I see a couple of posts saying wireless will introduce lag aka latency. While it will, this number or percentage is NOTHING to sweat. Day after day corporate wireless networks support huge amounts of VoIP phones. The only ways latency would pose a problem from wireless...
1. Piss poor setup/administration by the owner. (you)
2. Way too many device connected to a single AP. For a standard consumer AP, I could see it handling 10 or so without issues. High end APs do 20-30, so I think 10 is conservative for home APs.
3. The bandwidth available from an AP is getting utilized. i.e. you're doing file transfers on your LAN to/from a laptop using an AP and there is little to no bandwidth for gaming. In this scenario a wired connection would bypass the AP and help.
4. You have torrents or some other application/task sucking up your Internet bandwidth. In this case nothing would help since the Internet connection is the choke point.
Personally I have a simple 802.11g connection from my XBOX and I've used Netflix and their VOD/Rentals w/o any problems. I also stream videos all the time without issues. I'd PREFER a wired connection, but like most people I use wireless for the ease of setup.