mac pro 1,1 vs pc desktop amd phenom 2 x2 920?....

I have a 2007 mac pro 1,1 Quad Core" 2.66 is powered by two 2.66 GHz dual core Intel Xeon 5150 processors with 2gb ram osx lion and was wondering supposedly i can install some pretty good video cards like a gtx 660.

My current desktop pc has a amd phenom 2 x2 920 quad, 4gb ram and a gtx 560ti which performs little lower than a gtx 660.

So was wondering will the mac pro performance be better compared to the amd desktop I have? This is after i in a better card than the 560ti and more ram like at least 4gb. Thanks

The ATI Radeon 5770/5870 are good ards but with 2GB of memory that is a much more urgent problem. In my Mac Pro 2.1 days used 8GB, and now in a 2012 iMac 16GB. Better? Google Benchtests and see.