I have a NVidia GeForce 4000MX 128MB graphics card. Should I upgrade or is what I have good?
If it's working well, and your happy with the FPS and the graphics, it's not the most powerful card then keep it.
If your unhappy and want better graphics, switch.
Keep in mind that FSX will more then likely need a much stronger card so spend now what you can use later.
The next version isn't that far away.
I also have 512 MB ram. Will FSX work well?
You should have more ram for this version, who knows what type of memory sucker the next will be, if there is one this year.
As you have noticed, many of the members have 1 gig of ram.
It not only is better for the sim to run, but it makes a big difference in the rendering time of the textures.