It should be in the I/O ports section.
You'll be fine. All PCI-E x16 cards have to be the same width from the port to the back of the case.
I'm not worried about it fitting; there's no opening on that side of the case. I'm confused as how to I'll reach all the outputs.
Another consideration you need to make, is that your Compaq case might not be big enough to fit the new card into, they do not give you much to work with when it comes to oem cases. The gigabyte GTX 260 card your looking at takes up two spaces which that looks o.k. but the length is 10.9" long so you might want to measure inside the case before you order it. You'll also need a 2x6 pin power connector. Any newer psu should have one
Will do. I'll measure it. Thanks.