With the cage in place it seems that the
lian li PC-A05FN case can take graphics card up to 280mm long.
However, the GTX 460 1GB graphics card performs no better than the HD 6850 1GB and costs over £40 more. Therefore, I would strongly suggest reverting back to the 6850. If you can afford the extra for a GTX 560Ti - then I would go for it - though I would also be tempted to get a
HD 6850 2GB instead for a further £16.
As for the PSU, the CX 500W is a decent choice, though I would be tempted to get a
decent 550W PSU if you are going for one of the better cards.
The i3 2120 + Z68 combination does make sense if you plan to upgrade in the future and definitely want to overclock the future CPU (as sandy bridge i3 CPUs can't overclock at all). Though you may be better off (in terms of performance now) getting a cheap H61 board and instead opt for a low-end i5 2000 series quad core or even a second-hand i5 2500K. This option will cost about the same money and offer you a lot more CPU power now (though you won't get features like SATA3, overclocking and SLI/CF).