It also depends what frequency you want, I ordered the wmp600n (Linksys) it is 5ghz and 2.4ghz compatible and is 300mbps. This one is the only one I could find with 5ghz support.
Edit: to answer your question mbps does matter as long as you router can support it, for example if your router is only 150mbps compatible having a 300mbps adapter is pointless as you would be limited to whatever the lowest speed is (in my example 150mbps)
When MoM said if you need wireless n that means 150mbps and above (I think 124mbps might be included under n but im not sure). 54mbps is G.