No you can replace the psu, the psu is just screwed in there as if you would install your own, but then you pay extra for psu and it's not worth it, although the nsk4000 are great cases. they will set you back about 119 and it is called that nsk4480B series. Your best option will be just to look through newegg to find smallish cases which you like, or choose from the two trotter and the one i said.
SuperBiiz.com - Antec NSK6580B 430W Power Supply ATX Tower Case (Black)
this is the nsk with a quality 430w psu, that is good
, made by seasonic the earthwatts series are amazing for the price in bundles. but i gues you have already got a psu, or you want more wattage for future upgrades. So just keep looking for some small cases.