Is this a good desktop?

Depends on what you want to use it for. It is more of a mid-range gaming machine then anything.

Should I replace the gtx 560ti with 660ti
http://www.bestbuy.com/site/NVIDIA+-+GeForce+GTX+660+Ti...

do i need to get a new psu if i use 660ti

There isn't any real need to, but if you want a little more oomph then go for it.

And no, you shouldn't need a new PSU, but since it the PSU isn't listed in the system specs, I'll refrain from saying either way.