We start our clients business computers with an i5 and 8 gigs....just for basic stuff, and go on up from there. With just basic apps...there's a big difference in performance on identical systems otherwise...where some will have 4 gigs, others 8. I just got reminded of how big of a difference the performance is yesterday, as we onboarded a new medical office client. He had identical workstation (about 25 of them)...only difference was some had 4 gigs, others had 8 gigs, and the rest had 12 gigs. Sitting down at each and every computer....uninstalling :"junk"...cleaning them up, doing Microsoft updates, web player updates, and installing our agents...big difference in performance 'tween the 4 and 8 gig systems just doing those light things.
Lots of our clients run Quickbooks....and we ourselves run it (just upgraded our own office to Premier 2015 about 2 months ago). I've seen it so many times on different hardware..and see the difference in performance all the time. If it's a networked multi user version...you want it on a server with a fast disk system...I've seen it on all types of "servers"..and seen the differences in performance from the client side.
if your clients use it a lot and want it running good, and you want to give your clients a well running system...don't skimp.
If your clients balk at the prices of setting up proper systems to run it well..explain to them clearly the differences in performance...and how cheaper systems will run slow for them. It's a common line of business application..the users live in it all day every day (same with any line of biz app). Clearly explain how, for the life of the computers (say 3-5 years)...their "ROI"..Return On Investment...should illustrate on spending a bit more now on a proper system will yield good results over that life of the computers. Maybe some people think that Quickbooks normally runs slow...heck...lots of us call it "Slowbooks". But once you actually sit down and run Quickbooks on a proper speedy system...you realize that it doesn't have to be slow. And you'll be in pain if you have to go back and run it on a slow system.
It's not a hard sell.