Looking for a 1U or 2U rackmount server, ideally with support for RAID (S-ATA even better) and definitely with dual PSU support. Single CPU, even slow one, is fine. Reliability is the key.
The sunfire v40z is a seriously great system. Extremely reliable and fast as hell. If you have some bargaining acumen you can get it for $4k or so.
If that’s too pricy, the dell pe2650 isn’t too bad. That’ll run around $2500 with coupons or a dell biz rep. We’ve had some problems with the embedded PERC raid, and some of the drivers are flaky under (very) high load. I wouldn’t recommend it for oracle but for apache/tomcat/mysql stuff it’s great.