I've just realised that by running a second monitor from the one graphics card, I'm taking a big hit on RB 3D performance. I've unticked the 'Extend desktop onto second monitor' option so I'm just running the 22 inch widescreen by itself. The 3D window is much smoother and I can actually see the 3D models with Maximum set for Hardware flights. The PC is still gasping a bit and can't cope with anything other than Minimum for Network, but if I watch local flights only, it looks very nice :-)
I'm running the primary monitor from the HDMI output and the secondary from the RGB/VGA output of the NVIDEA GeForce 8500GT. I suppose that this makes sense really, it's just that I hadn't really noticed any loss of performance until I started using RB 3D - pushing the boundary I guess.