Found PSU on here:

http://www.amazon.co.uk/OCZ-OCZ-CXS500W-UK-CoreXStream-Series-Supply/dp/B007SU2QRK/ref=sr_1_1?ie=UTF8&qid=1393341034&sr=8-1&keywords=ocz+500w+psu

Pretty average I think, nothing great. But came prebuilt with my rig.

With that PSU, you shouldn't have to have problems with any of the cards in your budget or even above.

Take a look at the power consumption of a whole PC with different gfx cards.

http://www.anandtech.com/bench/GPU14/885

You'd be able to get one of the top of the line GPUs (so the 780Ti or 290X) if you wanted!

How is this one:
http://www.box.co.uk/MSI_NVIDIA_GeForce_GTX_750Ti_Video_Graph_1553589.html?gclid=CLK_mLfH1rwCFYcSwwodM0QAjA

http://www.novatech.co.uk/products/components/amdradeongraphicscards/amdr9270xseries/11217-04-20g.html

Buuuuut its AMD

AMD is my card maker of choice. With the upcoming release of Mantle, a graphics API that will make PCs work very similarly to consoles (more access to your's rigs "metal" compared to the restrictive and thus fairly antiquated DirectX), your rig will play games like never before. Also take into account that both the PS4 and XBO are AMD based architectures and you'll figure out that most games that will come out in the next few years will be optimised for AMD cards.

You probably wont need to upgrade your CPU/Mobo for the entire gen.

For GPU get (in price order)
GTX 770 2 Gb for 330$(or 280X if you can find it for that price)
GTX 760 for 250$(or HD 7950 for same price)
AMD 270X or 270 (non X) for less than 200$
AMD 265 2 Gb for 150$
GTX 650 Ti Boost 2 Gb for less than 160$
GTX 750 Ti for less than 150$

My sweet spot being the GTX 760 or HD 7950

I would suggest an EVGA GTX 770 if it does not go beyond your price range. There is not a single game out there that it cannot play at 60fps at 1080p with everything maxed to the gills (except Metro Last Light). They are also quite cheap right now (in the US at least). Just watch out though, all modern cards are massive nowadays so make sure it will physically fit in your case. 10.5 inches long is longer than you may think.

Maybe this could help:

http://www.videocardbenchmark.net/high_end_gpus.html

AMD or Nvidia. It doesn't matter at all, both have pros and cons.

If you want the best card for your money, AMD is the way to go. It's that simple.

If you want lower power consumption, this gen Nvidia is the way to go.

And if you prefer a quiet card, choose an AMD with an aftermarket cooler or an Nvidia one.

By the way, the 2 cards you have listed will exhaust the hot air from the GPU chip inside your case, so be sure that your case has enough airflow to dump that air out of the case or look for a card with a cooler that exhaust the hot air out of the case (reference coolers).

660 Ti boost 3GB /thread
how much does it cost around there?