I could be wrong but I think I remember Vivek saying he didn't get as good of results from the Alfa N card as he did from the b/g one.
I was considering getting an Alfa but I checked the connection on my internal Dell wireless 1390 and it's constantly at 54Mbps so I'll stick with this one. :-) (i was surprised)
If you're planning on doing anything more than monitoring (e.g. packet injection), then you'll want to make sure your internal card is capable. One of the reasons for getting an Alfa card is that a lot of internal wifi cards can't do packet injection.