Thanks, Sprogget and Bush5A.
I did start to believe the idea that SLi was purely for super high end, high speed, graphics - and eventually I had no real *need* for it.
The original PC - before the one in question - had a standard off the shelf AGP2 graphics card supported ny a 1.7gb CPU and, whilst absolutely fine for general PC use, gave me a frame rate of about 4 frames per second on Second Life.
I designed and built my super-computer ("Deepthought") with SLi in mind in an attempt to improve the in world experience. The SLi never worked, on three different serial numbers of the same, exchanged, motherboard, possibly due to an inherent fault with the motherboard, the graphics card choice and 64 bit Vista - never tested by the manufacturer - they used different cards and Windows XP to test.
Running Deepthought on a single, yet fairly powerful, graphics card supported by a 3.2 gb CPU gave me a frame rate of anything from 35 to 60 fps - which is higher than broadcast television (25 UK, 30 USA) and renewed my questioning the validity of SLi.
Now that I have traced the (different) fault on this particular motherboard (intermittent connection between PCI-E graphics card and mobo) and have to replace it with a different model (as commented by Sprogget), I wondered about trying SLi again - but not at the cost of dropping down to just one monitor.
The new PCI graphics card that I used to prove the fault with the PCI-E slot is now giving me a totally unworkable frame rate of just 2.4 fps - makes Second Life absolutely impossible.
Thanks again for the wisdom.
=^..^=