Right, first a short story: My computer decided to just black out and die the other day, and it seems my old ATI Radeon 4600 HD series graphics card decided to up and die (it was old. Like really really old. Like look at when that was released, old). So I am in need of a new graphics card for gaming.
Give me your recommendations as I have not kept up with graphics card news in ages, I have no brand loyalty, and looking for something newish but affordable as well (target price is ~$100), and something better then what I had before as it was ooooolllld and I was looking to upgrade anyway so now I have an excuse to.
I'd be wary of synthetic benchmarks and look at actual game benchmarks like Anandtech, TomsHardware, or other tech websites do.
Driver-wise, nVidia tends to be better with drivers and interact better with programs (for a while, ATI's drivers conflicted with making a call on certain versions of Skype on my box) but that can change from driver release to driver release
That being said, watch for memory bit size and overall bandwidth. The Ti550 has 192-bit memory bus, the 6850 has 256-bit bus. Both having 1GB of Video Memory, the 6850 has a larger capacity, so then it boils down to how effectively the cards use their available memory capacity. Thus, the 6850 starts off ahead naturally and even if it isn't quite as efficient as the 550 Ti, it has more room to work with.
I double-checked a comparison review between the 550 Ti and the 6790 (the step down from the 6850) and in an intense game like Battlefield Bad Company 2, the 6790 always stays ahead. Food for thought.
I currently have an ATI Radeon 5770 and although it's getting dangerously old (Battlefield 3 on high settings without AA) it should be relatively cheap. Certainly close to your budget.