Most current games work best on a fast dual core, but more are using the extra cores of a quad so for a new build I'd get the slightly slower quad.
As an upgrade to an older system the GTX260 or HD4870 is fine, but less good for a new build.
I'm not suggesting you sell your body parts or cut down elsewhere, but look towards the GTX275 or HD4890 if you can get either within budget.
Like the quad, the faster cards are geared towards future demands rather than current needs.
If you can hold off, the new DX11 stuff should be hitting release around October, perhaps you might want to wait until then and re-appraise the situation when some proper facts, prices and benchmark numbers are out.