Disclaimer: I do not work for Oracle, so my answers are very unofficial... Minimally, if I answer anything incorrectly, perhaps that will provide a starting point for Oracle staff. Let's start with something that can be debated...
No. of cases to be processed daily - 10000-20000
I assume that the No. of cases to be processed equates to the number of invocations of OPA... Minimally, I assume you will have under 100,000 invocations daily (or spread between 8-5). That is not a very high load in our experience as long as you don't try 100,000 in under 10 minutes over a slow network.
I assume the PC is dedicated to OPA.
OS: Microsoft Windows 7
Windows 7 works, but is not a server OS. In general, the numbers I am providing below should work for either IIS or a Java Server to host OPA at the loads above...
CPU Processor & Speed: ___i5______ [I have seen it run very well on less CPU, but with a Java server it is nice to have a quicker startup. Plus, if you are like us, you paid for OPA per CPU. We have not had problems with i5 or better on windows 7.]
RAM : ___4____ GB [Most PCs come with this. OPA is not a memory hog in our experience.]
Hard Disk Drive: _____10____ GB for OPA above and beyond the OS and App server [More space is used for log retention, than by OPA which I have seen installed in under 3 MB for determinations-server.]
Cache: __________ Whatever comes with the system is usually fine in our experience. It is more important to tune the system not to go to sleep.
Again, the above numbers are not official and only represent our experience on developer desktops with a full OPM install. We have load tested against these desktops during early stages of OPA usage, so we know OPA will perform very well. There are other variables, such as network, other software which might be running on the system, complexity of the rules, load balancing, etc.
Given the cost of OPA licenses, our organization justified the expense of better hardware simply to eliminate any risks with hardware. We especially made sure we had good server CPUs. Our general argument went "We spent $$$,$$$ on OPA, so we should be able to afford at least $,$$$ on hardware." Reuse of a $$$ PC that might make the $$$,$$$ expense not perform well is not worth the risk. We have other things to worry about.
Thanks PaulFowler for your inputs. For other developers/experts, hope you can continue to share your thoughts.
We have consulted our previous OPA trainor as well on what are the Hardware requirements for the OPA server, but said unfortunately there doesn't seem to be any document to that level of detail. They're not aware of any technical documentation on this topic other than the System requirements. However they advised it is worth re-posting this topic again to the development teams out there to confirm.