The scenario is a small company with a few hundred users. ISA Server 2006 (at time of writing) will be implemented primarily to restrict websurfing and various applications like instant messaging per user.
My experience with ISA server has been on HP Proliant DL360s and DL380s - is there any reason to reccomend some other server or configuration?
The DL360 is the least expandible, the G5 has one full-height slot to add a 4-port network card. But even this should be sufficient for the web proxy role?
If you only need ISA Server to act as a web proxy, you will either need one network port, if you only connect it to your private network, or two, if you put it on the edge of your network, with one internal and one external interface. Additional network ports are only needed if you want ISA Server to act as a firewall between multiple networks.
ISA Server doesn't by itself require a lot of CPU and/or RAM; requirements are also eased by it running only on Windows Server 2003.
One thing you should take into account is disk space: if you want to keep logs of what is passing through your firewall/proxy, you will need some; the same is true if you want to enable caching for web content. The best setup in terms of performance would call for three physical disks (or, better, RAID volumes): one for the O.S., one for logs and one for cache.
For comparison, we run ISA 2006 on a 1U Dual P3 1.2Ghz with a gig of ram and some 36Gb SCSI disks and apart from only having 10gb or so for cache, it's fine. And as Massimo mentioned, if you're just being a proxy between your network and your router, you only need one NIC.