Well, a new CPU will speed up everything not just games but that video card will hold you back in games. Not sure a fast video card will do much for speed on a slow machine. You'll be able to turn on more features and use higher resolution but it might not speed up much. It depends what features are already enable with your present video and what games you are playing.
When I had an AMD K6-2 400 with Geforce256 SDR (performs very close to an MX-400). Later I installed a Geforce2 GTS and didn't gain anything in framerates (about 24 fps). Place that same Geforce2 in a Duron 1 ghz machine it scored about 80 fps (1024x768, max detail).
I'd upgrade the processor first, though I'm not sure what your upgrade options are.
There's an easy test to determine which, the CPU or video, is limiting your system most. Choose a benchmark like Quake3 Demo001. Run it at different resolutions like 640x480, 800x600, 1024x768, 1280x1024. If the framerates are all the same then your CPU is unable to keep up with the video card. If the framerate starts high but continually decrease with each higher resolution your video card is the limiting factor. If the framerates start off level at lower resolutions but eventually start dropping at the highest resolutions then you have a close match between video and system speed and you would probably need to upgrade both.
<b>I have so many cookies I now have a FAT problem!</b>