Well yes you can replace PSU fans, but there are two concerns: 1) They don't always use standard fan connectors, usually, but not always. 2) I would strongly recommend against ever opening up a PSU unless you know EXACTLY what you're doing in there, you can get some very nasty shocks if you're not careful.
Replacement wise, if you're willing to spend that much I would highly recommend a PSU from Corsair, for example
this one
I don't now anything about that Ultra one you suggested so I'm not saying this is necessarily better, but the Corsair PSUs are very good, absolutely silent and hugely reliable. You probably won't get a modular one for that price (though look around just in case), so the Ultra has that advantage, but you have to decide how much you think you need that flexibility...
-- Thu Jul 23, 2009 1:45 pm --
Looks like that Ultra has had some good reviews, so I can't fault that as a choice if you go that way.
My question though would be do you need a 600 watt supply? Could you get away with less to save yourself a few bucks?