I have a Lenovo IdeaCentre K430 with an NVidia Geforce Gt620 graphics card. I would like to upgrade to an AMD Radeon HD 7750 but don't know if this is possible. If I got the GT620 model, am I limited to Nvidia cards? Or, can I still upgrade to an AMD card? In particular, the card requires CrossFireX, as I want to have two of them. Thanks for any help.
Yes, it's a desktop with a PCI Express x16 port, put whatever you want in there, well, within the limits of your power supply that is. Since the 7750 requires no dedicated 12V, you should be fine. As far as a second card goes, I think that motherboard only has ONE PCI Express x16 slot, so don't get your hopes up.
You can put in just about any graphics card you want. You are limited only by space. The design of some motherboards and cases can cause issues with larger/longer add-in cards (like graphics cards). The only other limitation is power. It would appear your K430 uses a standard ATX power supply, so even if it doesnt supply enough power, it can be easily replaced.