It looks like the video card you have linked is for a low profile slot. Your compaq has normal ones. An x1550 is better than what you have, but not by much. It's pretty much a slightly beefed up x1300.
For dual channel you want to match the size of RAM sticks in each channel (2x512, 2x256, 4x512, etc)
For dual channel you want to match the size of RAM sticks in each channel (2x512, 2x256, 4x512, etc)
