I up you with Roomba 805 at Costco $399, Li-On battery and 2 virtual walls. They had a $50 instant rebate back in Jan which I sadly missed.
My research says buying regular Roomba with other kind of battery and DIY yourself with an after-market Li-On not a good idea. Amazon reviews don't exactly confirm whether 650 comes with Li-On or something else. But if on tight-budget the 650 IS a Best Buy. I just prize built-in, designed-for use with Li-On battery. The other item I prize is, the 805 looks like it comes with the new kind of brush which requires less maintenance. Why do you buy a robot? so you don't have to do the work right? So another one up more for less maintenance brush.