Optimize performance of large library and masters, and hardware?

I followed some very useful advice here during the transition from Aperture 2 to 3, about a year ago. It dealt with keeping things running efficiently, when managing 100,000+ photos in your Aperture library. Defragmentation, eSata drives, referenced files, etc.

The best information came from a "Kevin J. Doyle". I'd be delighted to hear from him how he currently has things set up, including hardware. (Like me, he is a registered user, but I don't see any way to contact him directly via e-mail, and don't see any web page for him elsewhere. So this is a message in a bottle. . . .)

I'm currently wrestling with inadequate hardware (iMac 7,1, with 6GB Ram, and multiple hard drives via fw800), and suffering long hours doing basic housekeeping just to keep things barely adequate. For example, after I've done a lot of overnight copying and repairing of things, I still have to wait more than ten seconds for an individual image to load on screen before I can edit it. And if I use brushes, the resulting "processing" can sometimes take up to a minute before I see the particular effect of my brushing. I waste a lot of time. (I spend a lot of time keeping customers updated on my slow progress with their pictures too, hoping they don't get too frustrated.)

I know it's a common observation, that computer hardware and software must be routinely maintained (usually via other sofware, eg. Cocktail, Disk Warrior, etc.). I think I'm beyond the point I can eek out better performance from this generation of iMac. I'm thinking about the Mac Pro next. I'm not too concerned for now about the rumored end of Apple development of the Mac Pro. I probably should be, but I'm sure that anything I get hold of in the Mac Pro line from the last two or three generations would be faster and easier to use (organizing multiple hard drives, in particular).

I'm also keen to know how to organize my library better. I have 250,000 and counting photos, referenced. 2TB and more of masters. I work with thumbnails, but without previews (because they made the library too large to copy in a reasonable time, i.e. overnight via fw800, and did not seem to speed up the editing in any way I could measure). Currently everything is in one library. I've tried making a small "work-in-progress" separate library, thinking that might speed things up. It made no difference. Tried the same with managed versus referenced. No difference. Anything else I could try?

My masters are located on a very fast (raid-5) and large (6TB) disc, accessed by the library via fw800, because that's all I can get from the iMac. FWIW, the location of the masters does not seem to have anything to do with the editing performance slowness. I and others here ran those tests a while ago, when I switched from a managed to a referenced library.