Well, for the most part your downloads are going to be limited to you Internet connection. Unless you have a DS3 or better, FIOS...etc 11b would work. However, the speeds are assuming theoretical perfect connection.
I'd at least go 11g. PC to PC transfers will be heavily slowed by 11b and 11g will still be slow compared to wired. 11g will also allow you to utilize faster Internet connections such as DS3, FIOS and faster - without the wireless adapter affecting speed.
My '03 Sold.