Dual Monitor video cards for HP Pro 3400

I am looking to add a 2nd monitor to my HP Pro 3400 MT and want to make sure I get a video card that will not have any compatibility issues.  I'm not doing any gaming so I don't think I need anything super fancy and I would prefer not to have to upgrade the power supply.

So my questions are:

1. Is the HP Pro 3400 MT compatible witha PCI Express 3.0 card?  2.1 ?
2. How much power draw from the video card would require me to upgrade the power supply (my PC is stock, no added peripherals).  Is there a way for me to determine how much is being currently (no pun intended) drawn?
3. Are there any drawbacks to usnig the built-in video card for 1 monitor and a new, single-ouput card for the other (vs a dual-output card) ?
4. Anything else I need to consider when deciding on a new video card?

Thanks.