I have problem to setting up a 4 monitors computer. I bought a Gigabyte GV N750OC 1GI which can support 4 monitors simultaneously, but in my Window XP only can make 2 monitors active with Nvidia Control Panel... Should I upgrade to Window 7 to enable 4 monitors simultaneously?
Although you might run 4 monitors on one card (Nvidia MultiView) using Windows XP, you should definitely upgrade to Windows 7 to avoid further problems. It is very old and your videocard / drivers / Nvidia MultiView might not support 4 monitors.
See manual page 3 for supported OS: http://download.gigabyte.ru/manual/vga_manual_e_nv_104.pdf
IMPORTANT: For ALL (99.99%) of video cards, you CAN'T connect more than 2 legacy connectors (DVI, HDMI or VGA) otherwise you'll get blank screens on the other monitors. This means that for your Gigabyte GV N750OC 1GI card
NOTE: you may, however use an HDMI to DisplayPort adapter, since these are not legacy.
This is because videocards have a physical "clock" limits for old / legacy connectors.