Although it's a totally different product, there might be some relevance in the test results of various NVIDIA video cards with "Badaboom", a CUDA enabled program. You can see some figures in this article, which I believe may have been mentioned by a previous poster.
http://www.anandtech.com/video/showdoc.aspx?i=3374
Now whilst it would be impossible to make any direct comparisons, it could be that similar ratios of performance gain from different cards might apply to PD7. That might suggest the 8800GT as a reasonable candidate, at around $140, or the 9800GTX at around $180. If CUDA works in PD7, it would mean you could use a lesser cpu. If you're wanting quad core anyway, that might logically be a Q6600, giving a good balance of non CUDA performance and cost.
Of course if the promised ATI hardware acceleration works with the HD4800 series, it makes a Phenom cpu with a 4800 series video card look like a nice machine to me, since the Phenoms can be had at pretty competitive prices and AMD motherboards also tend to cost less than Intel ones.
I've emailed CyberLink to see if there is any info on GPU encoding with the HD4800 series. As I see it, that feature could easily be a deal maker for an increasing number of people struggling to edit AVCHD.