Yes your system is bottlenecking now and Will with future cards. Unfotunately GPU'S at the moment are very CPU dependant. If you have an Out of date CPU it can sometimes really hold you back...I noticed this especially since my Upgrade.
No its not a Rumour. Sometimes you can get some serious driver and registry conflicts resulting in Instability and OR BSOD. However a repair Install can Remedy these issues.
I also think it's a rumour that you need to reinstall when changing mobos. I've never been "verifying" my XP installation.