Question Feasibility of using an old system for video capture ?

So I have my gaming system that can be seen in my sig but what I have questions on is setting up an old PC I have for video capture and would like opinions and thoughts on if it would suite my needs. Also I am in need of information on capture cards.

Capture System Specs:
Motherboard: Gigabyte GA-970A- DS3P
CPU: AMD FX 6120

GPU: Sapphire AMD HD 4870 (can be changed out for either a Asus AMD HD-7950 or XFX AMD HD-7970 that I have on the parts shelf now)

RAM: G. Skill Ripjaws 8GB @1600MHz
HDD: Seagate 500GB prob. sata2 <-- Can be changed out for bigger from the parts shelf.
PSU: Antec EarthWatts ('20 ancient '07-'08) 650W PSU <--- that needs to be swapped out due to age.
Case: Antec 902

This system was thrown together for the youngest kid from some spare parts that were lying around and wasn't meant to be a powerhouse or gaming machine to start with. I'm under the impression that this will be good enough for just a capture machine then edit the captures on my main gaming system.

Questions:

Q1. In your Opinion will this old system be good for capturing video 1080p - 1440p?

Q2. Would a USB capture device work well with USB3 on this system ? This will dictate if I need to get an internal card.

Q3. Questions about capture devices and how they work, I've seen some USB capture devices with just 1 HDMI input. Do these need a signal splitter from the GPU? I can't see how it wouldn't.

Q4. On capture cards that have 2x HDMI ports I assume this is a pass through for the signal in and back out to the monitor. Does this create any lag in the signal from the GPU?

Thoughts and opinions welcomed. I will not be building a newer system for this, just using what I have and buying a new PSU to handle the 7xxx series GPU out of fear of how old it is.