Modernising and old system by patching it up with modern bits is seldom successful and you will probably, in the end, spend more money on improvements than you would on a completely new system.
If you replace your CPU then you'll find the graphics card doesn't do it justice. When you replace the graphics card, the sound card is inferior. Then you'll need more memory. After that you find your PSU is struggling to provide power for all this new stuff and it goes on and on.
By all means upgrade but don't do it by mixing old stuff with new.