Buying a new card ever week is not how to build a PC son, the 4870 is still one of the strongest cards, maybe only DX10 but DX11 has 1 improvement, tesselation, and if you knew what you were talking about you would know what it does and how it will make no difference in games when MSAA is used.
Wow, it's disheartening to see people so ignorant of the feature sets of various DirectX implementations make such absurdly incorrect statements about graphcial functions.
Tessellation and anti-aliasing have absolutely nothing to do with one another.
NOTHING. More importantly is that DX11 extends rendering functionality, performance, and flexibility by adding two programmable shader stages and one fixed function stage (for tessellation). There are many more features that DX11 adds, but there are far too many to list here, define, and explain. Especially since you can hit any tech site such as Ars Technica, Anandtech, Techpowerup, Guru3D, or Elite Bastards to get a full read-up.
While there are different types of geometry and tessellation formats, I'll stick to tessellation as a general term to keep things simple. Tesselation is all about adding geometry and/or making true bezier edges on rounded objects. It has nothing to do with edge smoothing, morphological, pre or post process, adaptive or anything else full scene anti-aliasing.
Example of tessellation vs no tesselation in the UniEngine Heaven Benchmark:

In the future, try not to act like you know something when you're completely ignorant with regard to the subject. It's one thing to ask questions, another to speak as if you're some sort of authority on the subject, when that couldn't be further from the truth.
There are numerous ways that DX11 can act as a faster rendering path to DX9. Especially if newer features like tessellation aren't utilized. Parallel processing to maximize use of the GPU as being fed by the CPU is one area where DX11 distinguishes itself, though the process must be inherently parallel in nature (all graphics cards these days are heavily parallel in nature). GPGPU applications, especially OpenCL improvements (supported by both nVidia and ATi - CUDA is nVidia exclusive).