A great budget card would be a GeForce GTX 560Ti, or for a bit more a GTX 570.
On the AMD side a HD 6850 or 6870 would be solid choices as well.
But there will be too much bottelneck with a cpu of 1066mhz fsb.(Depending on games). Even with a GTX 550i or HD 6770 will bottleneck in that system. So buying gtx560i is not a good idea.
So my suggestion is to upgrade system or ovcerclock cpu(it will redure bottelneck). Or else buy gpu like gts450 or 6670.