This is just my own datapoint. I built a dual Celeron 466Mhz system with 786MB RAM and two 80Mbit/s UltraSCSI IBM 9G drives for about $2300 which I run two OpenACS sites and also Oracle8.1.6 on occasion and it seems to handle it fairly well. I don't have a large amount of traffic but it does seem to perform well with the dual processors. I would stress that lots of RAM is a good idea :)
cheers
Jamie