What kind of content are you pushing through the marantz and to your projector?
Blu ray 1080p?
Cable provider 1080i/720p/480?
I can only see upscaling DVD content if you don't want to re-invest in buying the same movies over again.
I myself wouldn't process a processed signal, just send it natively to your projector....
You can try processing the signal from your gpu card - see how it looks on your screen....
then try just the processing in your marantz and see how it looks - then choose which looks better.....
Just my 2 cents
Denon AVR-3311CI, Denon DVP-602CI, B&K EX4420, B&K TX4430, Belkin PF60
Sony BDP-S5000ES, Silverstone HTPC (6 Tuner DVR & 10 TB Media)
PSB Stratus Minis, PSB Stratus C6, PSB Alpha S surround, James Subwoofer EMB-1200
Sharp LC-70LE755U, Harmony 880 Remote