Upgrading your graphics card now can improve game performance only to the extent where it gets bottlenecked by the CPU.
Since you will be using an i3 in the future, it won't be much of a waste to upgrade the graphics card now.
Get something like the GTX 650 Ti Boost, great value for money. You won't be able to fully utilize it with the Pentium 4, but you can transfer it to your new PC in the future. And when you add another similar card to SLI, you have a very powerful gaming pc.
But if you want to cut down on redundancy and cost for the most effective solution now, a HD 7750 is significantly more powerful than your 9600 GT, consumes less power, and can be had for less than $100.