If you haven't decided on anything yet, Guru3D just released their 3870K review today. Its a nice write-up, but I wish they had gone into a little more detail on the iGPU performance and how it compares to lower-end discrete GPUs.
From what I've seen, I think we can safely assume that the on-die 6550D has GPU performance somewhere between the 6450 and the 6570, probably a little closer to the 6570.
6450 - 160 stream processors @ 650MHz
6550D - 400 stream processors @ 600MHz
6570 - 480 stream processors @ 650MHz
5570 - 400 stream processors @ 650MHz
5750 - 720 stream processors @ 700MHz
Going from these numbers alone, I would say the 6550D would perform a little below 5570. Meanwhile, the CPU on the i3 is a bit stronger than that on the 3870K.
So, in summary, if you can get an i3-2100 + 5750 for less than the 3870K, it'll be better value, as it will perform better.