Sata 3Gb/s connection running at 1.5 Gb/s with new SSD installed

Right. I pulled off the SATA cables and couldn't see anywhere a drive jumper like those in the IDE drives of old could possibly have been hiding.

I've failed to find any reason my old SATA II motherboard is running this SSD at SATA I speed, but practically, it doesn't matter for a machine used mainly for email and word processing. And S.T.A.L.K.E.R. and its ilk.

I had a look at the MB manual. Try to disable functions like LAN BOOT ROM, IDE, SATA, serial, parallel and other hardware that you don't use. This will free some resources from the chipset.