What motherboard do you already have?
What video card do you already have?
What do you want to be able to do with the new video card?
Download and run CPU-Z (cpuid.com) to learn the manufacturer name and model number of your current motherboard.
You should have a socket 478 CPU with a Northwood core (confirm this with CPU-Z). That means you need to start your search by looking at socket 478 motherboards that support a P4 (not just Core Solo or Core Duo CPUs).
I only knew of three socket 478 P4 motherboards that had a PCI Express slot, they weren't the greatest boards, and they are getting difficult to find these days. You should still be able to find some good boards with i865 or i875 chipsets and AGP 8X video slots.
Another option that will allow you to keep your DDR memory is to move to a socket 939 AMD CPU & motherboard; but, if you are going to spend that much money, I'd just bite the bullet and build a complete Core 2 Duo system.