Ok, problem solved, at least for me, but I believe it will be for anyone who had this problem. It appears that Leopard, unlike Tiger, is quite sensitive with where the output from the graphics card is sent. There are two solutions for solving this:

SOLUTION #1:
Previously, when my system would hang on a light blue screen, my signal was being sent to a CRT monitor connected to a VGA connector. So by mere trial and error, I decided to see what happened if I connected my CRT monitor to the DVI connector using a DVI to VGA adaptor (delivered with my graphics card). Lo and behold, it worked. No TV-OUT, but it worked, giving me QE/CI on Leopard 10.5.2. My connected TV is not displayed in the displays pane, and, of course, no signal is sent to the TV, unfortunately, neither using Natit dual 0.2.0, nor anything else, unfortunately...

SOLUTION#2:
Reconnect your CRT monitor to the VGA connector and UNPLUG whatever adaptor you were using with solution #1. If you have them installed, remove any NVInject, Natit, Titan kext you may have installed.You will boot into framebuffer mode. Now install the version of nvinject for your card memory. Go to System/Library/Extensions and open the Info.plist inside nvinject.kext you just installed. Edit the NVCAP entry with the following value: 04000000 00000300 0c000000 00000007 00000000. This is the value for 7xxx cards I found on the nvinject site: http://nvinject.free.fr/forums/viewtopic.php?t=19. It should look like this:

<key>NVCAP</key>
<data>
04000000 00000300 0c000000 00000007 00000000
</data>

Save the modifications and do the repair permissions routine before rebooting. On reboot, you should have QE/CI enabled, and, as an interesting twist, your TV (if you have one connected) will be displayed in the displays pane as a VGA monitor, yet, once again, no signal is sent to it...

If anyone has any clue about how to enable TV-OUT on leopard, please share this information, it's the only (annoying!) thing missing from my Leopard installation.

PS: I have even been able to solve this problem and get my card detected using strings and modifying the com.apple.boot.plist entries. It works, the advantage being that you can bypass 3rd party injectors such as nvinject, natit etc. However, still no TV-OUT, even with that method... I'll try to post a little guide on how to do that, as well, but no time right now.