The A720is is a pretty good camera from the price. A650 is nice if you want more pixels and a 4xAA battery for flash (faster recycle times--the 2xAA A720 and A570 are pretty slow).
The A570is looks like a nice camera too (4x zoom vs 6x for A720is)... The A570 does not have anything to allow mounting of external filters/lenses like the A720 and other more expensive cameras have.
Otherwise, the next step up would be the S5is--but probably a lot more than you would want to spend.
I like my A720is. Yes it is light and plastically feeling--but, so far, has been rugged and takes very nice photographs.