Having recently gone from 4GB to 8GB on Win7 I'm going to share my highly scientific and technical analysis of the upgrade, and how it works with PowerDirector:
"I can't tell the difference."
Seriously. I don't notice any difference in performance. On the 64-bit platform PowerDirector 8 will allocate 1GB of memory during rendering, but appears to only use about half that. When I only had 4GB of RAM installed, it behaved the same exact way.
Ok. Not scientific. The extra RAM has other benefits, though. You can have more stuff running simultaneously. It probably contributes to system stability too, since it reduces memory thrashing while windows juggles lots of programs. I run a TV tuner card and record and watch HD TV on my PC while PD is rendering and it handles it no problem. Of course the video card is a big factor in that too, but that's my real world result.
On the bright side, it never hurts to have lots of memory, but if the choice comes down to adding more RAM or getting a better video card, I would go with the better video card.
It just doesn't use the extra RAM. Memory snapshot captured during render of 720P AVCHD:

This message was edited 2 times. Last update was at Sep 29. 2009 16:51
Intel i7-4770
16GB DDR3 1600 8-8-8-24
MSI Z87-G45 Motherboard
ASUS GTX 660 Direct CU II OC 2GB GPU
1 TB RAID 1 (mirrored) Drive Array
Several scratch drives for video, TMP, pagefile.