Is the GTX 260 too high for dual-core E6500?

An E6500 with a single GTX 260 should be fine. Especially if you can or have Overclocked the E6500.

Running SLI would be a waste however, so just get a single GTX 260. And also, make sure your power supply meets minimum requirements (for instance, it needs 2 x 6pin PCI-E power connectors).
 
Both the old C2D E6500 and the new P2D E6500 are definitely enough for the GTX260. I've seen people pair it with E5200s OCed to 3.0GHz (basically the same as a E6500), and you can OC yours to 4.0GHz easily on air.

I personally have a 4870 (equal to the GTX260, but more forgiving on less powerful cpus due to drivers) and it works fine with my E5300 with a small overclock.
 
As the "building a balanced PC" article by tomshardware showed, you need at the very least, a fast dual core w/ 6mb of cache (ie. E8400, preferably overclocked) or preferably a quad core for Nvidia SLied cards. (ATI Xfire is more forgiving due to different driver types)

A GTX295 (performs equal to two GTX260s) paired with a E6300 actually scored lower than a E6300 with a single GTX260 since the CPU couldn't power the graphics card effectively.