I'm surfing the internet through my wireless USB dongle right now, and I've dabbled in a few online PC games in the past, I have never experienced lag. My connection used to be 2 M/Bits but now it's 5.5 M/Bits. But the point I'm trying to make is this...
My PC's USB adapter cost only £9.99 (a sweex one from Play.com) and lag is non-existent on the PC side. Whereas my Xbox 360 is connected up "occasionally" with a 30 metre ethernet from the wireless router - which really is an eyesore in the living room!
And playing Soul Calibur IV whenever, there most definitely is lag. Going wireless won't make it any worse I don't think, just... I don't think there's any viable option for lag-free Xbox 360 gaming at the moment, it has nothing to do with your side. It has more to do with their server side.
Might be different for Gears of War 2 and Halo 3 etc - Halo 3 is usually pretty smooth.
Just my two cents (pence

)
Lol, anyways my point - Is £46 for a USB adapter worth it? £60 is a rip off and I guess hovering around £45 is easier to take...