NVIDIA launched its "GeForce GTX 550 Ti" GPU designed to target gamers who care about performance but are on a budget. It fits in right between the GTS 450 and GTX 460.
GTX 550 Ti has more performance than the GTS 450 but priced below the GTX 460 at $149. GTX 550 Ti isn't really considered the replace for the GTS 450 but rather "a step up".
GTX 550 Ti comes with DircectX11 support, same transistor-level changes the rest of the GTX 500 series GPUs come with that improve power efficiency. This essentially means that this GPU (and the rest of the GTX 500 series GPUs) can run at pretty high clock speeds but consume less power in doing so. Putting this into context -- GTX 550 Ti runs at a 900MHz clock speed (which can be overclocked to 1GHz)-- 120MHz faster than GTS 450, but consumes only a little bit more power than GTS 450 does. The performance per watt is much better with GTX 550 Ti than GTS 450.
Another big change with GTX 550 Ti is its GDDR5 memory controller, that supports "mixed density" memory chips. This allowed NVIDIA to put a 1GB frame buffer on this GPU using a 192-bit memory interface. Other GPUs that use a 192-bit memory interface are usually limited to 768MB of graphics memory. With the mixed density memory that NVIDIA is using on GTX 550 Ti, they don't have to boost the frame buffer to 1.5GB to ensure the full 1GB of memory graphics.
GTX 550 Ti is also packed with latest GTX technologies including SLI, PhysX, 3D Vision, and NVIDIA Surround. And, you can now get the new version of the 3D Vision wireless glasses kit on the NVIDIA Store, at a reduced price of $149 USD.