OK I am reviving this thread.
After toying with Geforce GTX 550 Ti for a few days to get HDMI working, I decided to return it to the seller because it was too much hassle, but I learnt a few things...
Now I got myself a very cheap (£12.50 GF 9500 GT 1GB Ram HDMI version) and it co-operates much better, plus requires less resources on the old Dell Dim 9200 I now use...
I am a bit late on Lion, Mountain Lion because my old rig was a Dim 8300 and the best and last you could put in is Snow Leo...
Now this is the stuff I did and learnt in the process...
1. NVCAP changes when you change input port (i.e. hdmi, dvi, vga, tv...) so keep track of ioregistry while using a port, it will be for that port...
however I stumbled on a little trick...
2. Using DSDT is ok so long as you know what to put where, and I got myself a DSDT for the HDMI... it worked somewhat with the GF GTX 550, but despite amending it accordingly did not work with the 9500, probably I am missing something...
3. By accident I used my old gfx string of my previous GF Card the XFX GF 8600 GT 256 MB RAM, and surprisingly together with the mods on DSDT (yeah I used HDMI with GF 8600 thru a DVI-I to HDMI adapter, and got it working smooth, so it was HDCP capable), it worked with GF GTX 550, and now is working with GF 9500 GT... amazing huh - I get hdmi working...
4. Next I thought this might be because the 8000 series are recognised natively by MAC OS X, so I tried the latest ever, which is the 8800, and yes it worked too, I get full hdmi on Lion, MLion and SnowL, provided I also use graphics enabler, otherwise it won't work...
5. It also works for dvi and vga so it is fun... and I can always modify the gfx string plist to report GF 9500 GT instead of 8800 GT...
So this is a painless work around to get HDMI through on the GF 9500 GT hdmi version...
Note: I do not know if GF 9800 is supported by Mac OS, but its gfx did not work - it seems the 8000 series only could work...
This method seems to work on other HDCP compliant cards like the GF 550 GTX...
Well this is me sharing this little lucky stumble... have a nice day/night