I just bought a new HP Pavilion PC with a Geforce GT 635 (2GB) graphics card. The quality of video is worse than my old PC which had a Radeon HD 5770 (1GB). I'd like to upgrade but I'm a complete beginner. What other cards would physically fit the slot used by the Geforce? (I know I need to check power supply but I just don't know anything about pin-types, etc). Any advice?