My wife uses QuickBooks on client PC's that are core 2 duo and some that are on i7's. She has found that some reports are faster to pull up on the i7's with SSD. I would say if they want to spend the money let them. No sense in fighting it.
Jesus, it's QuickBooks.... it would run just fine on a really low end Core 2 Duo as Deadius said above, hell it would run fine on an old single core Pentium 4.
And as he hinted at above, they'd be better off spending their money on an SSD - that's going to make a whole lot more of a difference in speed.
If they insist on the i7 and money isn't the issue give em what they want.
But whatever the course of action, make sure it has 8GB of RAM and some kind of SSD.
Personally, if they were all worried about QB performance, I'd go for a high clock i3, 8GB and SSD.
Unless they're doing other general work on the systems, no one will notice any other processor.
If they are doing lots of general desktop work and other web surfing, then yeah the i7 and hyperthreading will make the desktop experience better.
But for pure QB machines that don't do anything else? There's no point other than bragging rights.
Now if one of the machines is the server for a dozen QB client systems and they havn't told you that yet... yeah. Put the i7 in it... and at least 16GB RAM. (And make sure the network is FAST) QB client/server code and efficiency has always been horrible. All you can do is try to make it better.
I cringe when I see a dozen or more people on a local Windows QB server.