You can run any card you want, just make sure its running at x16. PCI-E is backwards compatible so you can run a 3.0 card on your 2.0 MB. Any POTENTIAL bandwidth limitations would be hidden by your CPU anyway. Youre asking for something that is compatible, and they all are with your setup (however you didnt mention what kind of power supply you have)
The problem you WILL run into is other components limiting your new GPU. If I were you, I would buy the best GPU you can afford with the intentions of eventually upgrading the rest of your machine. If that is not something you want to do, then buy the best card that wont be held back by your current configuration.
Id go with a minimum 650 GTX, but make sure you have at least a 400w PSU