So lemme get this straight
When I install a new ODD/HDD/Wat.
If it says for SATA Power like "P6, P3, ect"
Do I have to match up the numbers for the SATA Power label with the Number labels on the connectors.
No, it goes to whatever has the other gender of the connector, irregardless of connector numbers.
As someone with little knowledge, I am not sure how to compare cards and determine how one might be better than the other. The few things I see in the comparison is that the core/shader/effective memory clocks are lower, the CUDA cores are a bit higher, and memory size is different by one gigabyte. Also the obvious fact that one is a 550, and the other is a 560 (but somehow interestingly cheaper).
How much would these differences affect performance, and would it really be noticeable?
The GPU in the 560 SE is practically a rebranded 460 SE, which can actually be brought up to the speeds of a 460 with proper overclocking, will give you a noticable difference in most modern games compared to the 550 Ti.
This pages shows that the 560 SE ranks a higher G3D mark compared to the 550 Ti (224 marks difference and 10 ranks.) Barely noticeable, but still better for the price.
would I still be able to use the unlocked one?
Yes. The unlocked one I believe allows overclocking but does not force it, you can run it at base clocks.