January 7th 2014 Edit
I currently have a PCIE x4 Intel I350 NIC in a PCIE x16 slot meant for graphics on a Intel DH77DF motherboard working without a problem.
The original post below stating that "as long as it fits it should work" still stands. It's still possible that some motherboard manufacturers have a PCIE x16 slot that won't accept anything other than a graphics card. I think this only happened in the early days of PCI Express, and modern motherboards (say Core 2 and newer, and definitely Core i-series and newer) are pretty flexible in what you can use the PCIE x16 slot for.
Also, make sure you have on-board or on-chip graphics, or a video card somewhere. Most motherboards won't boot without video. Although, sometimes you might be able to configure the BIOS to ignore the lack of video and continue booting.
I figured I'd return to clarify my answer after encountering this very situation myself :)
Original Post
Initially I'd say there's no real difference between the PCI-Express slots and as long as it fits there's no problem.
However, the problem is if the PCI-Express slot is specifically labeled for video card. The tomshardware link shows the user placing the TV Tuner card in his second slot, not the first. The first slot (in your case, your only slot) might be graphics only.
Some point to yes. Some point to no. It's hard to tell and seems like it's up to how the motherboard manufacturers decided to integrate the chipsets.
PCI-E 2.0 devices are fully backwards compatible, so you should have no problems. The hardware will only function at PCI-E 1.0 speeds, though.
They probably recommend you don't use them in first generation slots for performance reasons, but the GTX550 shouldn't come anywhere near the maximum speeds of 1.x. I have seen both 8800GT's and 9800GTX's working fine in PCI-E 1.0 slots.
Best Answer
X16 is the width of the slot and the corresponding amount of bandwidth available to and from devices connected to that slot.
In your case you should be able to install any PCI Express 2 card in that slot. X16 is the largest of the slot options and will handle all cards of smaller size as well (X1, X8).