What machine to buy or build?

What machine to buy/build? This morning, I found myself with 10 instances of Chrome, Outlook 2010, QuickBooks Premier Accountant 2015 and QuickBooks Premier Accountant 2016, Lacerte 2015 aand Lacerte 2016, Intuit QuickBooks Pro Timer, ScanAll Pro, and 3 .pdfs open. Whenever I get my desktop loaded up like this, there is almost always a problem with performance. This morning the machine hung up up, again. Soon I'll have to have the personal property software running too. In case you haven't figured it out, I am a sole practicioner CPA.

Is processing power my problem or memory. I seem to think it is a combination of both. Only have 8 GB of memory on a 4 year old i7. Are the # of cores of the processor more important than the speed? Some of these applications take forever to start as does the computer to boot or restart. Would I notice the difference between a SATA drive and a M.2 PCIe SSD. Would I notice the difference between a i7-6700 and a 10 cored Xenon. Would I notice the difference between 16 and 32 GB of the appropriate memory?
Almost forgot: presently running two monitors. need a third and a fourth would be nice.
 
You may not have a hardware problem at all, it may be that your Operating System is just getting "bogged/slowed down" from age and usage. Have you done any regular file maintenance - such as defrag the hard drive(s), clearing out caches and temporary files, running a Registry Cleaner, run a hardware check on the hard drives, run memtest86 to check the ram for errors or failures, checked for malware / viruses, run updates on all the software and drivers? Also, when was the last time you physically powered down the computer and opened up the case to clean out dust? I only ask, because you did not mention it.
 
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