PCI-E 2.0 should be fine with a single GPU card.
ATM,your graphics card is you major limiting factor.
You need to consider that its probably best waiting until Skylake,PCI-E 4.0 and DDR4 at the end of next year or early 2016.
An overclocked Core i7 950 and a decent card should serve you well
IMHO.
Sure,the latest Core i7 is technically faster,but I would always try out a new graphics card first and overclock your CPU.
Upgrade when you actually hit a
noticeable CPU bottleneck - that way you get the latest tech too.
Edit!!
Do you know on Ebay there are cheap six core,ex-server Xeons being sold for under £100??