Is my gaming and trolling pc any good? How does it compare? What upgrades would be good?

I have a full ATX case with a full ATX main board from ASUS. There is an INTEL Core 2 Duo 2.4 ghz processor and an Nvidia G Force graphics card with 256 megabytes of vram. The system has 4 Gigabytes of system ram and includes two hard drives.

One hard drive has a capacity of 350 gb and is the boot drive, the other has a capacity of 500 gb and is the multimedia drive.

The system has a DVD burner and and from drive (both 48x), and has a Zip drive as well as a tape drive and both a 3.5 inch and 5.25 inch floppy disc drive

There is a 10/100 ethernet card in the system as well as a 56k modem.

The system is water cooled and has GamerGirl BathWater circulating in the water cooling kit. I also added the rgb color changing lights for inside the PC case.

The system presently runs windows Vista and performs very well.

I buy one new hardware/software item every year to upgrade my rig.

Some of my past upgrades were adding an additional 2 gb of ram to now have 4gb

Adding a second hard drive

Adding the water cooling

Adding the case lights and GamerGirl bath water for improved performance

What upgrade would be most effective this year?