well as said, it will bottleneck at stock for sure, but the upgrade from a 550ti to the 290x with the bottleneck u may not see the huge difference from it being bottlenecked to a cpu that can handle it. Again as said overclocking will help quite a bit, i would at 3.2ghz my x6 would prob just run my 7970/280x same thing lol but at 3.7ghz performs basically on par with most 7970 benchmarks. Obviously the 290x is a big difference from the 7970, but if u have an overclocking board and get above 3.6ghz and near 4.0 it should help quite a bit.
Also the bottleneck will be less at the card will prob throttle cause of temps with the stock cooler lol, would recommend waiting til aftermarket designed coolers are released or add a arctic cooler...