Short answer: It depends.
Long answer: It mostly depends on how many super trends you have.
Check out page 5 of the PAC Display Users Guide. If you are in PAC Display Configurator, click on Help -> Manuals -> PAC Display Users Guide. (If not in config mode already, you will find the manuals under the Windows - > All Programs -> Opto 22 -> Manuals).
Some other factors to consider;
Page 35, multi monitors.
Page 97, the 'Always in memory' option for each window in PAC Display.
Page 195, how much data of the log file you want to keep in RAM before writing it out to the disk.
Page 206, how much data you want for each super trend to buffer in RAM before its written to the disk.
Page 240, single or multiple instances of PAC Display Runtime running on the one computer.
Page 289 How many alarms you want to buffer in RAM before writing them to disk.
Page 319, how many events you want to buffer before writing to disk.
Start with your OS of choice minimum amount of ram, then go up from there.
Since RAM is so cheap, and these days most versions of Windows knows how to use it, there is no danger in having more than enough.
Bottom line. For a pretty small project, with a few of everything, then your basic computer will be fine.
If you have more than say, 50 windows, half of which are super trends.... spend the 50 bucks and get some more RAM.