How do I know if the video card I will buy is compatible with my motherboard?

I have plans to buy an ATI Radeon HD3850 512MB DDR3 AGP. My motherboard is an ECS P4M800PRO-M2 1.0 and the compatible memory card is a DDR2 PC2-5300. Since the video card I'm about to buy is a DDR3, does that mean it's not compatible with my motherboard? Or is it still compatible because it's an AGP and my motherboard is compatible with AGP video cards?|||Just look at SYSTEM requirements on video card. Or if buying in a computer store environment ask the geek squad. Best wishes.|||i think its still compatible since the video card RAM is independent of your systems actual RAM and the slots match