Problems with a server class machine.
- video, just a basic video. Not much room to put a better video in it if you need better video performance.
- What OS you going to run? Depending on what it is, you will not be able to utilize the processor(s) effectively.
- Disk controller? SCSI, SATA, SATAe ... SCSI disks are smaller than the SATA disks currently and are more expensive. With SATAe, you will need some additional parts before you can hook up your disks.
- NOISE, little screaming fans.
- Cost of running it. Depending on what it is, it could be not very power efficient and will result in a higher power bill.
- Expansion slots? nope - not here.
Xeons may seem to be P4's but the only similarity is that they run the same instruction set. Their pipeline structure and cache is very extensive compared to a desktop P4. The transistor count is about 4x more, more power.
At one time I had a match set of 4 Xeon processors and I did a pass on trying to use them for a machine.