Well, the plot just thickened by a large degree. I agree that you're right in that I need a second (machines) opinion and I just realized we DO have a more modern computer than the clunky old P4 - my wifes computer, I just never use it so it didn't spring to mind before.
I unplugged the GTX 460 and slipped it back in its anti-static bag and at the same time I plugged the old GTS 240 back in because even though its fan sounds like an eight motor propeller bomber on the runway I still wanted the computer somewhat operational while I was tinkering with the other one. So, I booted up the computer with the GTS 240, reinstalled one of the nvidia drivers for it, and out of curiousity decided to have a look at the games again before moving on.
The result: Same antialias problem, regardless of game, regardless of settings, now using the old card.
So basically it now seems safe to say this isn't a hardware issue of the Gigabyte card because now the disease seems to have spread to my GTS 240 as well. The strange thing is though, I absolutely KNOW the Gigabyte installation caused this issue because I was playing Crysis with beautiful antialiased graphics ealier that very same day (and since over a year back) on the GTS 240, then I shut down the computer and installed the GTX 460 only to have the antialias function suddenly be broken at the end of the installation process - and permanently broken it seems
I realize this is a longshot to even ask but, does anybody know if there's ANYTHING that might have gotten installed from the Gigabyte installer CD that could be causing this conflict? Something that might have managed to stay on the computer even through all my nvidia uninstalls and reinstalls?
For example, when I just now glanced down at the CD I noticed "Yahoo companion Toolbar" on the cover. Being severely allergic to all kinds of toolbars and particularly Yahoos malwarian sh*t I would have never wanted this installed, but when I looked in the programs list - there it was! After a quick delete of Yahoo I also looked for another suspicious "GIGABYTE Easy Boost software" noted on the CD cover, but that on the other hand seems to NOT be installed - at least I can't find anything by that name.
Since the only clues I have now are that A) it seems to be a software issue after all and B) it started when I installed the Gigabyte card, I can't imagine what else the cause could be besides the CD. Well, unless the card installation somehow physically broke something on my motherboard that just happened to result in no more antialias in any games but that sounds a bit too unlikely to consider.
Any last wild guesses?