What are the specs of that Dell?
If it's a newer one I'd be careful about blindly changing out fans and such. They seem to be pushing the thremal envelope with the factory installed ventilation and changing to lower speed fans may put you over the top, temperature wise.
If it's an earlier version you may have some working room. The earlier Dell's (PIII and back) seem to be over-fanned for what's inside of them. Be careful though, those Palo Alto cases don't have very good airflow so you may need to keep some fairly decent fannage in there to keep things cool enough.
The earlier Dells are also a lot more proprietary than the newer ones. You won't be able to swap out the PSU, for instance, unless you do your home work and even then it might not work.
Some of the Dell fan connectors are also proprietary, even on the newer Dells. They seem to be tied into a (non-disableable of course. This is Dell afterall!) BIOS circuit and the system won't boot if you just unplug them and try to replace them with something different, even if you adapt the original plug to the new fan.
Main Box: Intel i3-3225, Intel DH77EB, 16GB Corsair RAM, 256GB Samsung 830, SS360GP PSU, CM PS07 case.
Music Server: Intel DH77EB + i3-3220, 2xSamsung 2TB F4, Pico PSU, Fractal Define Mini, 2xScythe Fans @250 rpm.