The think the Radeon HD 5770 would be the closest thing ATI has to the GTX 550ti. A comparison Like above stick with NVIDIA.
Haha good man Seriously though, I would still stick to NVIDIA - they have the best value to performance.
Nvida fanboism? As already said 5770, 6970 or a 6850(since you can get one for £108 and it kicks the daylights out of the 550ti)
When you take into account that ATI's do not work and NVIDIA's do - no, it's not. You can spend £120 on a graphics card, or £120 on a piece of plastic. Your choice.
wow, are you serious ? let me guess, you had a ATI card and it died/or something so now ALL ATI cards are crap. now what would you do if your 560 died/etc ?
Looks like you missed the recent compilation chart of price/performance ratio. The card that falls in the number 1 spot for price/performance ratio is the HD6850. Can do nothing but recommend it, Ive had no troubles with my Asus direct CU model. If the OP wants to stick to nVidia though, I'd say stick with the tried and tested 560ti