First off, don't get the 911 Carrera wheel, it's PS3 and PC only.
Put simply, it's a device that allows you to use PS3/PC wheels such as the G25, G27, or DFGT on the XBOX, which has no support for these wheels, and pretty much only supports the likes of the Fanatec GT2 or CSR wheels. I use a G25 w/ XCM F1 converter mainly because I have both an XBOX 360 and PS3, and use both for driving games. Another option could be using a Fanatec GT2, which supports both the PS3 and XBOX 360,, which is the one I'd recommend (I know it's $250US, but it's worth it). You will need to get pedals as accessories though, for some reason they don't come with the wheel itself. The next step up would be the CSR Elite, but that isn't completely released yet. Trust me, read the details before you buy, that 911 Carrera isn't going to get you anywhere if you just use an XBOX.