You can run Microsoft SharePoint Portal Server 2001 only on Windows 2000 Server with Service Pack 1 (SP1). The new product doesn't support Win2K Professional, Windows NT, or any form of clustered Windows server. Because of the lack of information from real-life deployments, formal sizing information for SharePoint Portal Server is difficult to come by, but you should take the following considerations into account when you buy hardware on which to run the new server.
First, include a second CPU whenever possible to handle peak demand and background tasks such as crawling and indexing. The second CPU also provides greater flexibility over the long term. Any of today's CPUs will deliver adequate speed; the important consideration is the number of available CPUs.
Second, SharePoint Portal Server shares common database technology with Microsoft Exchange Server. Judging from Exchange Server's Extensible Storage Engine (ESE), which likes memory and makes good use of available system memory, I suggest that you equip SharePoint Portal Server machines with at least 256MB of RAM. Treat that figure as a minimum requirement, and go for 512MB of RAM whenever possible—apart from providing better performance, such a system can remain in production longer.
Third, SharePoint Portal Server's databases use a transactional logging model just like Exchange Server's databases do, so you need to identify likely sources of I/O and ensure that important files are correctly protected. You need to separate (i.e., place on separate physical drives) the transaction logs from the SharePoint Portal Server databases, and keep logs and databases away from Win2K and application binaries if at all possible. Failure to do so creates a situation in which a disk failure could render logs and databases inaccessible and could lead to a loss of data. To protect the SharePoint Portal Server databases and get the best performance, place the databases on a RAID 0+1 or a RAID 5 volume. Mirroring can deliver adequate protection for the volume that hosts the transaction logs. For a departmental system that serves as many as 200 users, a dual-processor, 800MHz or better server with 512MB of RAM and separate volumes for the binaries, logs, and databases is sufficient. Remember to include an appropriate disk controller.
Like Active Directory (AD), SharePoint Portal Server uses circular logging to reduce the number of transaction logs that user activity generates. This step also simplifies management because you don't need to worry that the disk holding the logs will exhaust available space if a backup hasn't been performed. The Msmdback.vbs utility, which comes with SharePoint Portal Server and which manages backup and restores, simply deals with files during restores and doesn't attempt to replay transaction logs. (The utility restores a server to the state at which you took the most recent backup; intervening transactions are lost, so you still need a solid backup regimen.)
Before you decide how to install your servers, read Microsoft's performance recommendations for SharePoint Portal Server deployment. For example, although no technical limitation exists on the number of documents you can store in a folder, Microsoft recommends that you store no more than 500 documents in a folder. (You can have thousands of documents in a folder, but a client might struggle to fetch the information necessary to display the Web Part that lists the documents.)