The secret to a good experience on Second Life is having your graphics card support OpenGL Natively.
Just because a graphics card is super-fast at, say, Skyrim will mean that it's good for Second Life.
Almost all graphics cards support DirectX, which is what the majority of video games use. Second Life, however, uses OpenGL for 3D rendering.
I use a GForce GTX550ti which supports OpenGL Natively, and I can run my graphics all day long on High or Ultra, at great framerates (30-70FPS). I specifically bought that graphics card because of the pricing (about $230) and because of the OpenGL Support.