I was going to get a Palit GTX 560 Ti Sonic for $315, because at 8.1' long I think it will fit easily (reason for the high price is because I cannot find one in Australia so I will have to ship from Great Britain)
or am I safe to go for longer cards like the Gigabyte GTX 560 Ti SOC at 9.5' long for $300, or even a Radeon 6950 for the same price?
It will fit your board the question is your case, you can find out using a tape measure. Looking at the CPU specs though I believe those cards will be a little much for the CPU and you would need a new PSU as well.
My case has more than 14' from the rear output of the Graphics Card to the end of the case.
I am worried that the overhang of the Graphics Card past the mobo base and into free space in the case might be an issue.
Also on my mobo there is a cable running from either the orange or cream coloured socket to the right of the picture I linked. It looks like any Graphics Card that goes over it will touch it.
Will that impact the Graphics Card in any way?
I know my CPU and RAM are outdated and will bottleneck almost any card I get, which is why I want a future proof GPU so that when I make a new build next year, I won't have to get a new GPU (and perhaps SLI later on).
As for my PSU, I'm not sure what you mean. When I bought it I though I was getting a good one with a 500W sustained output.
I though GTX 560 Ti's and 6950's were recommended 500W PSU?