I am currently selling my Pentium 4 PC and I am not even using it since it is aging.
I think that it is better for you to first upgrade your CPU and Motherboard like getting Core i7 or wait for Core i5 in summer.
Your Pentium 4 system could bottleneck your new video card's speed. Your P4 CPU and PCIe 1.0/1.1 bandwidth are the things that can bottleneck the speed of your video card since the newer video cards would be using PCIe 2.0 bandwidth.
For the video card, you would wanna wait for DirectX 11 GPU which could come within only 3-5 months so you can do a bigger step upgrade and since DX 10/10.1 is about to be obsolete. However, DX 10/10.1 GPUs will still work with future games since they would still be compatible but it won't allow you to play games in DX 11 mode since you cannot use full DX 11 features, in other words, it is like you are playing another DX 10 games and not DX 11 games with the current generation DX 10 GPUs. It is also like you are stuck in time with DX 10 ability so that would not be something good and waiting for DX 11 is better. In order to play DX 11 games, you need DirectX 11 video card so basically, DX 11 GPUs could future proof yourself for the next 2-3 years before DX 12 comes out which would save more money and it's more convenient as well as working 100% fully well with future games and future products that might also need OpenCL for a longer time.