Can I mess up the system by testing a second graphics card?

bump

Well, you could definitely run into driver conflicts. As far as if the card will mess up the slot, that would be hard to say. Unlikely, but possible if the card is totally bad.

Please don't bump your own threads...

Apart from the obvious need to install new drivers for the ATi card and then uninstalling them afterwards, you should be fine.

Besides what was already mentioned, (uninstalling, reinstalling drivers for the cards) it can also affect the graphic settings on any programs or games you have setup and optimized the way you like them with current GPU. You may get the "new graphic hardware detected" thing when you launch your games again, and have to restore all your graphic settings to where you had them previously.