No worries mate.
No need to feel stupid; if you've got a question, these forums are one of the best places to pose them.
SLI is simply when you take two or more Nvidia GPUs and put them on the same motherboard; connected
by a little "SLI bridge" that comes with the GPU. This makes the GPUs in question function more like one GPU,
yet providing more power. Some GPUs scale better and worse, when you add more. For instance if you've got
a GTX 670 and it provides a 100FPS at 1920 x 1080 resolution (A.K.A Full HD) in a given game, it might provide
50 more, if you add another GPU. Thus it "scales" pretty well when you add another. Other GPUs might only give
a handful more FPS or barely any. Adding another GPU requires a motherboard which is SLI or CF ready. It also requires
more power; which was why I mentioned the thing about the PSU in my last post. It can also generate more heat and such.
Generally, adding two or more cards together (Some motherboards allow for as much as 4 way setups) can add more power
to your build. What's neat about it is that you can take two midrange GPUs and SLI them and thus rival or surpass the performance
of a more high-end and expensive single GPU.
As for CF, it refers to CrossFire X and it's exactly the same deal as SLI; Except it pertains to GPUs made by AMD; such as
Radeon 7970, Radeon 7850 etc. etc. etc.)
I would recommend upgrading the case too, if you feel like it. Some cases are better suited than others for various
things. That case from the last build, is meant for quietness mostly. Cases are personal preference too; so it's kind of
like a placeholder for you to pick a different one, if you'd like.