Kirby,
Granted that all of the this is anecdotal and not scientific, but the results are interesting. There is no way that I can afford a new Mac and certainly not a new MacPro. It does appear, however, that with a 100 MB file, I can reach the limits of CPU and buss speed. (The GPU was updated to a 5770 and so I am guessing that that is reasonably healthy as Apple still offer it with new MacPros.)
I am going to make some new Versions, one each of a TIFF on the SSD and on the HD and start tearing them up with Activity Monitor open. I was pleased to notice that, when doing a mass rerender of all my previews, all four cores showed even activity. Would lead me to believe that with Lion, at least, the work is getting spread around all of the hardware.
And least it sound like I am whining or trashing the SSD:
-- Aperture start to display first screen is under four wall clock seconds.
-- ALL scrolling, open project, shift views, etc. is absolutely instantaneous. Nothing takes longer than a second.
-- Interestingly, slamming sliders back and forth (e.g. exposure, shadows, etc.) displays results on screen faster than the trailing "Loading" and "Processing" wheels, which seem to lag by about one second. Real world adjustments, made by nudging the sliders and considering the effect are instantaneous.
All of this on a healthy, but old machine. Sooooo, I would postulate:
-- Get yer RAM up to the 8 GB reagion. (At some point, there may be dimishing returns unless you really like to keep a lot of programs running. All of the "tests" cited above were with Safari and Mail runnnig.)
-- Really old MacPros will benefit from a newer video card. Ironically, I noticed this when I was running on a 4 GB RAM Mini - it was actually faster than the MacPro with 5 GB of RAM and the original video card.
-- After that, you are more likely to be bound by processor speed and buss throughput than you are by HD read/write time. There might be some benefit to putting all of the masters on a dedicated drive, but, using iDefrag, I note that while the masters are fragmented after a reading in a full CF card, at worst, the files are usually only two extents and iDefrag's "Quick-On Line" setting will fix that very quickly. Does a similarly good job of cleaning up the Thumbs and Previews.
Of course, commen sense reminds that none of this means anything if your OS is messed up or your HD is too full.
Bottom line: SSD are expensive and it can take a bit of work to shoehorn your OS, Apps, and Aperture Library onto one, but the overall increase in system response in noticable and nice. Frankly, the hardest part was converting to Referenced Masters and fighting those parts of OSX which are designed to guide you into a nice, need, single disk system. (I am still learning how to clean up the Sidebar.)
As always, comments welcome.