How to get a real time HDV preview? Very disappointed

A bit similar to the previous post: I've experienced bad preview performance myself for a long time on my former system (like dv that played from the TL with 95% cpu usage, and HDV at 3 fps) This week, finally I took the big step. Bought brandnew hardware, one of the fastest QUAD processors you can find, great motherboard, etc, and a 8500GT videocard.
It's so much faster then my AMD3200+, that I didn't doubt for a momentthat now, finally, I would enjoy realtime full size playback of HDV from the timeline at Preview-full setting. Just like, judging from forums, a lot of people seemed to have.
No.
Brand new Quad Q6600 system, fresh Vegas 7 install. It gives me a framerate of around 10 fps at preview-full.
Have I allways been wrong assuming people could actually see their HDV footage during editing in Vegas, in the quality of the source material? Is it just not possible at all?
That's a well hidden secret, then.