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With respect to the experiences of others, I don't think it is so clear cut.
First, look at Adobe's GPU FAQ and see if the filters you actually use on a regular basis are even GPU accelerated.
Next, here is the closest thing I could find to an apples-to-apples benchmark covering GPU performance on CS6. Notice the marginal difference between the Mac Pro using the old nVidia GT 120 GPU vs. the one using the much newer, much faster AMD R5870 GPU.
Finally, look here to see the relative difference in "3D performance" of these GPUs. The 5870 is clocking in about 5x faster than the HD3000, which actually beats the GT 120 by a few points.
And then once you have viewed the third link, go back to the second and look again at the marginal difference in speed gain when using the new powerful GPU vs. the old weak GPU. I'm just not entirely convinced that a typical user will see tangible benefits from installing a dedicated graphics card in their system vs. the "fairly competent" HD3000.
Basically put, I think you will see significant acceleration by enabling OpenGL acceleration and using any supported GPU. You can get it faster by getting a dedicated card, but there are significant diminishing returns.
I'm not going to say that it won't be better with a dedicated card, but how much better it is really going to depend on how heavy your workload is, and what filters you are using regularly. And also consider that you may not see noticeable benefits from a dedicated GPU unless you invest in a higher end model, and not a cheap $100 entry level one.
I will also say that the new CS6 supports OpenCL, and as such I would recommend getting an AMD card which supports that feature. That supposedly brings about additional acceleration benefits. And lastly, getting a dedicated card also gives you the opportunity to edit in 10-bit color mode, if you are running CS6 on windows and have a monitor capable of supporting 10-bit output. (To get 10-bit output you will have to buy one of the "professional" line of graphics cards- i.e. the FireGL or Quadro series.)
Just my $0.02. I often hear people quip about how high end dedicated GPUs are way better for photoshop and that everyone needs one, but I have never seen anyone back it up with definitive proof. Only a lot of "it seems faster/smoother/better to me!" Here I have shown evidence that may suggest the contrary- if anyone would like to provide some facts to dispute what I have found after a brief 10 minutes of research I would be more than willing to listen.