There are no specific hardware requirements for P4P. However, for a Perforce Proxy Server for the use of more than a few hundred users a server class machine is strongly recommended.
The job of the Perforce Proxy Server is to pass information between Perforce clients and the Perforce Server and store copies of the files not already in the proxy cache on file sync or submit.
For example, if the file is in cache the Perforce Proxy Server will decompress the messages from the Perforce Server, write it to disk and then compress the message and file before sending it onto the Perforce client.
For best performance the Perforce Proxy Server host should have:
A fast local network to the local Perforce client hosts.
Adequate disk space to store every revision in your depot likely to be used by the P4P users.
A fast multi-core CPU to be able to compress and uncompress files for multiple users quickly and concurrently.
Note: Unlike the Perforce server, the amount of memory is not as critical. Memory should be sufficient to uncompress or extract individual revisions from RCS files. Likewise a fast disk subsystem, while important for faster client access, does not require a higher end solution. For example, a RAM SAN would not provide much more benefit for the additional cost over a fast SATA drive.