Well, after much sleuthing via Google and various technical forums, I finally got the hint at what is up. Things were definitely pointing towards Nvidia drivers until I read a couple of posts about how DPCs can be affected/masked by a myriad of issues from OS settings to BIOS settings to loose cables to poor driver implementations.
One such OS setting was with regard to Vista by default enabling AHCI mode. I tried to find reference to such a mode in BIOS but its not applicable to Nvidia based motherboards because I believe it only applies to Intel based mobos.
The solution was to disable AHCI in Vista by flipping this setting to zero in the registry:
\\HKLM\System\CurrentControlSet\Services\msahci\Start: 0
Voila! Transfer speeds went from avg of 5-10 MB/s to well over 50 MB/s sometimes bursting to over 100 MB/s! Nothing like solving something you know is wrong with hard sleuthing work! Thanks to TwoGuns who helped me start thinking about AHCI more with his focus on enabling AHCI via the BIOS - which made me realize I didn't even have such a setting to begin with (http://www.vistax64.com/vista-hardware-devices/99457-sata-esata-hard-drive-speed-poor.html).
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