I have a S5000PSL that had X5148's with big heat sinks and no fans. The previous owner just jammed a fan on top of the fins of the heat sink. I bought a pair of X5355 and now I'm really in need of better cooling. There are still plenty of LGA775 coolers for sale but I can't tell if they'll be enough for this CPU, nor can I tell if I'll be able to mount two of them side-by-side as the CPU's sit fairly close to each other.
It looks like there was some kind of duct but the prior owner snapped off part of the plastic connectors and I'm not sure how to mount it or if it's even possible to use it.
The boxed CPU comes with thermal solution. There are two different thermal solutions, passive (heatsink only) or active (with fan). It depends on your chassis which thermal solution you should use.
For more details, please see the Spares, parts list, and configuration guide.