First off: you're sure they're not memory constrained, correct? An SSD will help if the slowdown is happening because they're overcommitted and paging to disk, but putting more RAM in the machine will help a lot more. I'm amazed at how much memory AJAX-heavy web pages can consume, so "office users" sometimes trip over this despite not doing anything you'd associate with workstation-style work.
While my instinct tells me to just go with the 840 EVO, It almost feels wrong to put a drive with that kind of speed in a machine that will only utilize about 1/2 of it.
Are they regularly doing 100MB+ disk operations (streaming, file copies, video editing, etc?) They probably aren't -- that's an unusual use case (and typically ends up spec'd with larger drives) -- and if they aren't, then the throughput
speed of the SATA interface isn't going to make any real difference. What will make a significant difference is the very low access latency of an SSD vs an HD, and that's largely independent of the SATA version. In other words, most users will actually utilize closer to 90% of the real benefit of an SSD, no matter what kind of connection it is on.
Flying Fox wrote:
Coincidentally, I'm visiting my elderly aunt at the moment and will be putting a 256 GB MX100 into her Wolfdale Core 2 Duo tomorrow, and I gave very little worry to the fact that it is SATA II-only (I was happy to see it was already in AHCI mode and the current HD was already aligned correctly, as that will make a bigger difference in real-world performance and saves me a bunch of futzing). I'll post back later on the results -- based on a similar operation I performed on a Turion-based machine a couple of years ago, I expect they'll vary from unnoticeable to dramatic depending on the operation.