My replaced my stock video card with a Radeon R6770 (trying to get sharper image/graphics). But after following the installation directions, the monitors won't come on...it's like they go into sleep mode. If I put the old video card back in, it works fine. Why wouldn't the new card work? The new card supposedly needs a 450W power supply; my computer has what I believe is a 300W power supply. Would this cause the card to not work, or do you think the card could be defective...especially since everything works fine when I reinsert the old card...?
Yes, the 300W power is probably the cause why your 6770 does not work. It can not feed the 6770 enough power. Your old graphics probably is a weaker card that consume less power & the motherboard is able to supply all the power it needs.
But the 6770 needs more than what the motherboard can provide so you need to connect additional power to it via a 6PIN PCIe connector. I am afraid you either have to upgrade the power supply or exchange your card to a AMD 7750 which is a bit weaker than the 6770 but will not require a PSU upgrade for you.
What is your power supply brand & do you have a pre-built PC?
The Radeon HD 6770 requires a 450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended. Are you sure that one of your 6-pin PCI Express connectors can supply ABOUT 75 watts.