The performance hierarchy is GTS 450 -> HD 5770 -> GTX 550 Ti -> HD 6850 at stock performance. Keep in mind that higher end GPU’s require at least 1 PCI-e 6/8pin connector, though most manufacturers will include an adapter to convert 2 molex connectors into 1 PCI-e 6 pin connector, also make sure your PSU has enough power to run the GPU. And make sure you have room in your case for a dual slot GPU, and can fit the length of the new GPU.
[i]A note on the 400 series, the GF100 GPU primarily have problems with power consumption and heat in comparison to ATI/AMD’s HD 5000 series, the GTX 480 being the first to be looked at and the worst example of Fermi as far as heat and power consumption goes.
The GTX 470, which was generally overlooked, was a much nicer card, though it still ran hot, it would generally stay between 85c and 94c during moderate to intense gaming, as for price, in the US the Price/Performance compared to ATI/AMD’s HD 5850 was far better by June of 2010, you were able to find sales that included MIR’s that lowered the price of the GTX 470 to $300, the HD 5850 usually averaged about $315 to $330.
The originally flashed down and later crippled GTX 465 was also a very poor example of the GF100, and was quickly replaced with the 1GB GTX 460 GPU which uses the GF104, it performed on the same level, but was cooler, more power efficient and cheaper, and is the most popular of the 400 series.[/i]