I believe it will be a long time till we see something like the RSX from the ps3 be put into a computer. From what I have read it works in the ps3 because the cell B.E. can pump out loads of data at a time and the RSX can keep up with this data transfer rate.
I believe that the closest thing to being able to use something like the RSX is an overclocked quadcore but we still have a long way to go. You still should be able to get ps3 like graphics from a tri or quad sli setup. The other thing engineers have to look at besides clock rate is just how many shader operations the GPU can do at a time. The more shader operations it can do the more powerful the GPU is. I hope you get what I mean.