I don't recall all of the features of the 901 but it is a few models back from the current line. I think the main distinguishing feature of that model was that it has NetTune and is the most powerful model of the lineup. Other than more power, it is identical to the TX-SR801.
The 7xxx series has always been the best bang for the buck in Onkyo's lineup, IMO. Being the newest of the model line it has video transcoding from composite and s-video to component, 7.1 and multiple zones, supports all of the current codecs, 24/192 dacs, and the configurability features that they leave out of the 5xx and 6xx series - like the ability to set power on volume, preferred codec per input source, lip-sync adjustment, etc. The only thing it lacks is HDMI and for that you have to get the 802 which is about $200 more.
It is also THX Select certified if you care about that...