128 refers to the memory on the graphics card itself, GeForce refers to a large range of nVidia graphics chip based graphics cards.
128mb of ram onboard a graphics card is generally adequate for FS2004, while 256mb cards are available.
The actual thing you are concerned with when buying a graphics card is what Graphics Processing Unit (GPU) it has installed. This determines the type of card it really is.
I'll take a stab in the dark and assume you have a nVidia GeForce FX 5200 128mb 8x AGP card.....
The 5200 number is the GPU, the one we are concerned with.
The current FX range of nVidia GeForce GPU's are:
5200
5600
5700
5900
and the new one is a lot different and very powerful, the
6800
All those GPU's have variants and they are only nVidia chips, ATI make a range of RADEON GPU's as well, as do others.
It's a complex subject and needs a lot of research, you can read through the threads here to get an idea.
The best graphics card for FS2004 is the latest and greatest and most expensive that money will buy.
I'm sure someone else here might tell you how good they are, but I haven't seen one yet......


Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&