Question about Parallels and ATI Radeon 5770

Ok, so I have a Mac Pro 3,1 here. I'm upgrading it to 16GB and two 7200 RPM 3TB drives which I'll run in Raid 0.
It has an ATI 2600HD with 256MB VRAM.  This card has two DVI ports.
I just purchased an ATI Radeon 5770.  I will use it with two Cinema Displays (DVI connections).
To connect the second display, I have the Apple Mini DisplayPort to Dual-Link DVI Adapter at Firmware 1.03.
My question concerns Parallels 8.
My Windows 7 VM will be a heavy-use VM so I want snappy performance.  I was planning to dedicate 8GB RAM, half my CPU cores to it, and 512MB VRAM on the 5770.
What I was wondering about was the possibility of leaving in the 2600 HD and using its GPU to support my Parallels VM.  Is that feasible?  Practical?
Thanks
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Dump the 2600XT (put it on the shelf, don't use it)
Install Windows natively first, then you can use it both ways, natively and dual boot, plus in a VM when that works.
Another option is 10.8.2 supports more Nvidia cards and more then the 1GB.
Don't use green 3TB in an array.