Core 2 Duo Compatibility with VGAs ?

Which core 2 duo?

doesn't really matter, I've got the lowest core2duo e6300 1.86ghz in one of my computers and it can overclock as high as 3.2ghz on air cooling, becoming one of the faster core2duo models... so it really don't matter which model it is. These cpus like to overclock and it's so easy to do so.

I am thinking about buying a GTS 450 , also I don't know if there was another ATI GPU better than it and not to be an overload , i know there would be a bottleneck with the GTS 450 on a C2D
PS: I chose a GTS 450 not a GTS 250 because of the DX11 so any suggestions

You could look at the Radeon 5770/6770

May a GTS 450 work - although an HD 5770 is way better - ?
A computer sales man told me that a GTX 550 can work !
Wouldn't their be a bottleneck in the two cases ?
But the problem of me is that I can't compare an ATI with an Nvidia , Because i find a low end ATI with a stream processors of 400 or 80 and a middle or old High end Nvidia with a 96 or 128 Stream processors , so I go to Compare sites or hierarchy chart like in this site and so alot of specs in ATI differ completely than an Nvidia's .