Ehem, as an owner of the full Netburst collection- Willamette, Northwood, Prescott and Pentium D, I feel I should chime in:
Clock-for-clock, Prescott is in most benchmarks slightly slower than Northwood- but by a negligible amount. In real world terms, it's a wash between the two. So you'd see the same improvements as going to a 2.8ghz NW.
The 533mhz Prescott might just run on your Compaq board as it has no HT I think. You might need a bios flash, and the cpu might not be correctly identified, but I've heard of such things working. It's the 800mhz HT Prescott that's difficult to find compatible old boards for - due to the HT feature.
Prescotts do run very hot- think somewhere around 20c over a comparable Northwood. One redeeming feature is that at 3.6ghz, the extra cache and high fsb wakes them up and makes them faster than Northwoods. No NW can ever touch 4ghz, which many Prescotts can- though at the price of very high power consumption, heat and attendant cooling requirements.
But a moot point for a server, since you'll not be doing any oc'ing. At stock, a copper core Intel cooler will do the job nicely.
If you got the Pressie for free, I'd say just give it a shot.
