Okay, there are _many_ issues with the idea of just upgrading the video card.  I have looked into all options, and will list them here if I need.  But put simply I am hoping someone has direct, first hand experience with it.  I agree it's completely expensive.  But if it allowed me to continue using triple monitors while racing, and using my PC as I do now - then it'd be worth it as the "last resort".
To answer your question though, I'm on XP.  Beside that I'm not a fan of AMD/ATI cards/drivers.  Although I have put serious consideration, and almost purchased a 6950.  In the end, my research has shown me that eyefinity does not work with XP.  And I am not upgrading to Windows 7 on my current machine, as I already feel suffocated by XP (as far as memory goes).  I only have 2gb, and can't (and also don't want to) get anymore as my PSU wouldn't handle it.  Long story short, my system is ending it's termination date and I don't want to upgrade unless the upgrades move on.
I didn't explain that well, so just trust me when I say TH2G is (I believe) my last resort to iRacing in triple monitor.