54Mbps will be fine for playing online. The place that is the bottleneck for playing games online is your internet connection. I'd bet that you have 6Mbps or 8Mbps if you have cable internet. So it doesn't matter how fast your wireless is as long as it is faster than ~8Mbps.
Here's a long explanaition that you can skip if you need to.
There are some other things that could affect your playing but you shouldn't have to worry about them. Performance will go down if you have many devices connected wirelessly at the same time. This is because most wireless equipement only has one one radio so it can only send or recieve to one machine at any time. This means that if you were downloading a huge file and sending a huge file you would only get 27Mbps upload and 27Mbps download over your wireless (which would still be knocked down to whatever your internet connection speed was in the end anyway). If you had two computers downloading and sending full blast your 54Mbps would be split into fourths... Also if you have a not very strong wireless connection many routers will slow down the speed for better reliability.
In summary wireless should be plenty fast enough for xbox live. Even 802.11b wireless at 11Mbps works fine and is most likely as fast as your internet connection.