Are active PFCs a common feature in new PSU's?
In Europe (EC) certainly: they were hard to find a year ago but now most/all psus have it. It you are not in Europe I have no opinion.
does anyone think it's worthwhile to go 400W in terms of future computer needs?
No - unless you have an immediate need my opinion is wait. If you buy a 400W now trying to 'future proof', by the time you need the 400W you may discover your 'old' psu doesnt have the "whatever it is you need connector". Certainly if you are buying a psu now, and all else is equal, then take a 400W in preference to 300W. Except bearing in mind that the psu is around 65-70% efficient means (assuming 65%) a 300W psu has to be fanned to clear 165W of heat, a 400W has to be fanned to clear 215W. Which means an equivalent 400W design needs a lot more airflow (= noise) and you are at the mercy of the temp sensing fan design how much of that noise remains when you are only using a fraction of the peak power output.