The term overfilter crops up now and again but it could mean many things. There is mechinical filtration, do we really need to filter down to 1 micron or less? Porbably not, but is there any harm in doing so? Possibly... such a filter would probably fast clog and need more maintanence. There is bio filtration, and like rbishop says, there will only be a certain amount of bacteria based on the bio load - we could fill up a 50 gallon drum full of bio media but that doesnt mean its going to be full of beneficial bacteria, or be beneficial to our 10g aquarium versus a handfull of media. Having a little extra is called insurance though, incase the bioload were to change. Theres also chemcial filtration. But what a lot of people infer about overfiltration is about the flow rate. There is a limit on how strong a current fish will be happy with, depending on the fish of course, and depending on how that flow is delivered to the tank. So my point is that there are many aspects to 'filtration' and to aks what is too much filtration, well that depends on what your talking about and a few other details... just some food for thought.
And no, the baskets dont make a filter work better, just more convenient. Its not like you have your choice of baskets for the same filter (usually), the bigger filters come with more baskets. I mean look at the Filstars, the number (XP1,2,3,4) is how many baskets there are and each one has a stronger pump. Its not like you can get an XP3 with 1 large basket instead of 3 normal baskets. But no, just the # of baskets doesnt mean much without additional info.
