The hardware requirements are not as large as you would think. The most important aspect to consider is the RAM that is available for Squid. RAM is important because each object in the cache requires a small amount of memory. Generally, 32 MB of RAM are required for every GB of disk space. If you run out of memory there will be a significant reduction in speed.
The other major consideration for Squid is disks. The faster the disk read and write the faster Squid will operate. Usually it is a good idea to consider SCSI for disks on a proxy server just because of speed. The other advantage that SCSI has is that it can access 7 different drives allowing for multiple reads and writes without a slowdown in access. If you are using ATA drives and have multiple drives on one channel you will find the system has to wait as it can only access one drive at a time. However, SATA drives or even some ATA drives are increasing in speeds and are much cheaper.
There are a number of variables that impact the speed of Squid and the hardware that is required. One variable is object size. The larger the object, the more memory is required per object so this may increase memory requirements. The second variable is the number of users that are on the system concurrently. This is a large variable in that the difference between 5 users and 105 users is considerable. The point is, plan for growth and estimate high for concurrent users so you do not need to come back later and upgrade.