Tripple buffering will just give you extra performance if you usually use vsync, just follow the link in the op to find out how to force it.
"Multisampling" refers to a type of antialiasing that only applies to polygon edges, its the standard type of AA that you'll usually find in your ingame settings. Your GPU has dedicated hardware to accelerate this and its very fast.
The problem with using just standard MSAA is that a lot of other things can cause aliasing. Shaders and specular maps in particular can cause some nasty aliasing within a texture which MSAA can't do anything about. Alpha textures (commonly used for foliage and fences to fake polygon detail) can also be a big source of aliasing, thankfully your driver has a setting to deal with this. ATI label it "adaptive" AA and Nvidia, transparency AA, there's two settings, transparency multisampling won't have much of a performance impact but it can cause a dramatic improvement to image quality, I recommend forcing it globally in your drivers/nhancer. Transparency supersampling has a severe performance cost which limits its usefulness, though the results are pretty damn nice
Supersampling is AA that is applied to the whole rendered image, the GPU is basically rendering the scene at 2 or 4 times the size (depending on the amount) and then downsampling it to your monitor's native resolution for the ultimate image quality. This is the same technique used to create console "PR" shots. Unsurprisingly the performance cost is
very high but since the results are so good its often worth forcing in older games where you've got plenty of spare performance. To force this with Nvidia cards you need to download nHancer (linked in the op), something which is well worth the download as its much easier to use than Nvidia's standard driver tool.
nHancer's website goes into much more depth than I have and explains some other AA modes like CSAA:
http://www.nhancer.com/?dat=d_AA
