I agree with you that given the cooling performance engineered by Shuttle on the SX38 is not exactly quiet, I own an SX48 which is identical to sx38, and I can make a few recomendations
The original PSU is a 450 watt unit and is fairly quiet and rather than replace it with another unit and all agravation involved in modifying another unit to fit, I would mod the original psu and case to improve the cooling which also lowers the noise from the psu.
The mod for the psu, requires you to remove the exit grill onthe psu fan, And I would also sugest you use a rounded metal file to enlarge the hole on the rear of the case where the it lines up wth the psu fan as for same strange reason shuttle decided to make the hole a couple of mm smaller than the fan.
In the above foto you can see the psu inside my sx48 with the grille that needs to be removed and the circular hole that needs to be enlarged slightly to match psu exit hole.
I also removed the 2 60mm fans above the psu as In my experience thay are noisiest fans in the shuttle, they whine even at low rpm and move very little air, I even tried running them with out grilles but they are still noisy.
My next sugestion is you improve the cooling to the shuttle ICE cooler, although with the original fans there isnt a huge difference, its worth reversing the flow of the fans so they blow air in rather than out. I found the optimal arrangement to be a 120mm fan and a 92 mm fan. more info regarding this mod can be found here.http://www.sudhian.com/index.php?/forums/viewthread/106275/