Asymmetric speeds (faster download than upload) are pretty common, and work fine. The most important factor for online gaming is really latency - not an advertised speed by an ISP, but might be able to get anecdotal info from other users on that ISP.
Depends on if you are the server for your 360 game. A low upload means that you are going to provide a crappy experience to clients connecting to you. 1.5mbps should be fine for 16 players though, which is a big reason as to why you see 360/PS3 games cap out at 16 players as a standard. Really though, the game should pick the person out of your party that has the >5mbps, lowest average latency connection to be your server. In theory at least.