I've read this thread with interest as I have a similar situation to OP.
Current specs are
CPU: Intel i3 @3.07GHz
Memory: 3gb DDR3
OS: win 7 home premium 64bit
Video: ASUS HD6770 1GB
Seat: custom built cockpit
Wheel: Logitech GT Driving Force
I run at relatively low resolution as don't have an hd display yet. I use textures full, cars high, circuit details med, low sfx and med shadows, 16x aniso and environment reflections on. HDR on and fxaa on, no AA though it absolutely kills it (together with high shadows)
I keep visible cars to 10, maybe less and have set max framerate to 62
A most circuits I find I can keep up the 60fps rate, dropping at start maybe. Some tracks are heavier on FPS but its usually driveable.
My point is you can get decent performance with avg set up. As an aside, PCARS butchers my machine and its 20-25fps at min settings and is awful on my system
However, as with the OP, is chucking money at something like a gtx560ti going to get me anywhere? I'm planning on boosting the Ram to at least 4gb anyway but do I need to step into the £200 plus gpu range to get to the next level of performance? Or will i run out of CPU before then?
Any advice grateful received