A Pentium 4 can bottle-neck most modern games. Unless you intend to play games that P4s can run, IDK why you'd want to buy a new graphics card. I don't think that any game that a P4 can run would benefit from more than say a Radeon 7750, if even that, at least not greatly so.
No Pentium 4 has DDR3 RAM. You need to provide more details. What you want the video card for, your PC brand/model, your power supply brand and model.
All LGA 775 Pentium 4s support DDR3 so long as their motherboard supports it. There are many such motherboards from the later part of the Core 2 era that support P4s and DDR3 memory. I can agree with your question, more information would be helpful.