There is still a sliver of hope that 3 X 8GB modules might work in the 4/6-core 2010 Mac Pro. If so, 24GB would pass a critical threshold, at least for my work.
To be clear, most users will find 16GB adequate, more than ample. But for users with big jobs (myself included), 16GB is not enough. It all depends on what you do. And a Mac Pro should be "Pro", not crippled to 16GB, because needs can change over time.
Solid state drives RAID-0 stripe to the rescue
Not just any drive of course, RAID is very demanding of SSDs. You’ll want a Sandforce-based SSD, like the OWC Mercury Extreme Pro RE. Note that Apple’s SSD is NOT qualified for RAID-0 striping (“Solid-state drives are not compatible with the Mac Pro RAID Card in either RAID or Enhanced JBOD mode”). Although that statement implies that hardware RAID is involved, it’s really about software or hardware RAID imposing severe duty requirements: RAID-0 striping imposes much more demanding usage on writes (small writes fragment SSDs internally).
Fitting dual SSDs and four hard drives
In my Mac Pro, I’ve pulled the optical drive, and I use the two SATA connectors in the optical bay to host dual 200GB SSDs as a 400GB RAID-0 stripe, good for ~500MB/sec. For system/applications plus scratch space plus anything else that needs speed. It really hums along seamlessly! That way, I can still use the four standard drive bays for 4 X 2TGB drives. (note: on 2006-2008 Mac Pros, there are two extra SATA ports which can be routed into the optical bay so that no need to remove the optical).
But you don’t have to pull the optical drive. You can use the lower optical bay for one SSD, and a standard drive bay for the other. That leaves three bays for hard drives, which you can configure as you please.
Why is this good? Because if you run low on memory, the system uses virtual memory to read/write to and from the boot volume. Using an SSD offer 4-6X the performance for virtual memory of a single hard drive.
Store your Lightroom catalog and previews, Photoshop scratch or anything of that nature on the fast SSD-based volume, with your big image files on the hard drives.