Hello. What model is your new cameras? Yes if you want to be able to switch between cameras you need a capture card/usb connection per camera. If your new cameras cannot connect like usb (webcam device) you need capture cards. The most cheap solution is to buy hdmi to usb capture cards that will import in vmix fullhd video from your hdmi cameras in mjpg format but you will not have the 100% quallity. For best result you can buy a card like blackmagic decklik that has many models according to your needs
As for GPU. What is your pc specs now? My experience says that if you want 2 cameras and a proper 1080p streaming and or recording a GPU is a must have. Even the old gtx 1050ti will perform better than the internal intel graphics in 1080p 2 cameras streaming