The fans I replaced are the pair of 60mm at the back of the case, above the power supply, being the smallest fans, it seemed a good place to start (as small diameter fans are normally the noisiest).
I'm certainly no expert, but I imagined anything would be better than the supplied fans, so I used the following
http://www.akasa.co.uk/akasa_english/spec_page/fans/spec_ak_186_l2b.htm
It should be noted at this stage that *all* fans in the SD37P2 are 4-pin PWM variety, not standard 3 pin fans. For these back fans I used the spare motherboard 3 pin header (of which there is only 1, I used a splitter), although I could have simply used a molex/3-pin fan adapter (one of which is supplied with the fan).
Also worth noting is that there is only just enough clearance to fit these new 60mm fans as they are 25mm deep, the originals are only 15mm deep. However I am working towards removing the back HDD tray by consolidating my drives into 1 high performance unit. Removing the back HDD tray should help airflow a lot.
The CPU fans are 70mm and 92mm units, I've not as yet sourced good PWM replacements for these, I'm reluctant to replace these with normal 3 pin fans. I suspect I'll end up buying 1 or 2 expensive CPU coolers to steal the fans.
Overclocking.... at 'stock' speeds (XFX is factory-clocked) I get around 9500 in 3DMark06, so I see little point myself. Yes I've read about overclocked units, but I've also seen watercooled shuttles and shuttles with the gfx card 'hanging out' of the case.
I suspect you can get away with some overclocking simply by running fans on "high", but as you'll be aware this isn't really practical with the stock fans.