We're running NW 4.2 on an HP lp2000r w/o problems. If you're running the standard configuration (i.e., no additional RAID cards, NICs, etc.), you shouldn't have any problems.
We're actually running ours with an Intel NIC in the PCI slot (instead of the integrated NIC) and an Adaptec SCSI card to handle our tape changer.
The only problems I've encountered:
1. I had to play with the IRQ settings because I had two add-in devices (tape changer and NIC).
2. I had to find the adaptec SCSI card drivers outside of HP's support.
3. Right now we're having probs with the tape changer - we can use one tape at a time, but the autoloading function isn't working properly.