A user of the application Ubuntu would need to install the Nvidia driver because default driver that comes packaged within the Ubuntu application lacks the ability for 3D graphics acceleration.
The only 3D accelerated (what I assume you mean by "functional") drivers for NVIDIA cards for Linux are for x86 and x86-64 computers. The only support for Nvidia cards elsewhere would be the nv driver (which is already included), and only supports 2D acceleration.
I would reccomend NVIDIA but they are very similar.
The operating system Ubuntu is a free platform that is available in several places on the web. The best place to download it from would be the official Ubuntu website, which would also provide tips and answers to any questions you may have.
Clinton
Ubuntu comes precompiled. Although you could compile it from source, that would take days, and would be pretty silly. To compile programs on Ubuntu, first "sudo apt-get install build-essential", then run "./configure" and "make" in the directory where you have the source code.
You can install Ubuntu easily on most PCs using their CD.
Nvidia GeForce 6100 GPU. Definitelly. :)
Ubuntu is safe. The majority of viruses and other malware on the internet are written for windows, so they wouldn't affect an Ubuntu computer. However, ubuntu is less user friendly than windows, so I would be careful and make sure you have a good backup for your windows computer.
The 9500gt is better, but still you would not get satisfying performance with it. Go for 9600 or 8800. 8600 is fine but still not that satisfying.
Normally the 6000 GT would be, but the 9400 GT a lot Newer than the 6600 so its overall faster.
Yes, you can. You will need to partition your hard drive because Linux has to be installed in a separate volume. I would install windows first and then Ubuntu.
Ubuntu server is a free download service. This is a very reliable program and has helped thousands to stabilize their computer programming. This would be a great asset.