Dark Souls 3 on this PC

http://www.systemrequirementslab.com/cyri/requirements/dark-souls-iii/13089

^ you can check here yourself even automatically but it doesn't look like you can ...
cpu probably barely cuts it, and gpu even further from it > still in 720p 30 fps probably yes

You can. I ran it using 560se (which is a bit better than 550ti but under regular 560 and 560ti) and i5 2400, but had to play on low/medium and lower resolution (like 1366x768). Still had some problems with 2nd (or 1st, depending on how you cound) boss because of "lots of fog" but managed to beat it anyway before I upgraded my card to gtx950.

Don't get confused by the lack of the game in my library atm, got removed couple of days ago :(

Game Debate: Can I run it

This is way more accurate than SRlabs. Use this and see

Thx
For me enough 720p on Mid

i think your main bottleneck would be the 4Gb ram. as most OS need 8 to run smoothly

Oh
Can i put on this http://www.ascendtech.us/ibm-lenovo-g31t-lm2-lga775-motherboard_i_mb4ibmg31tlm210.aspx beter CPU?

Try overclocking the CPU. Otherwise it's not worth spending any more money on a 775 system.
Your biggest problem is your GPU. You need a better one. An AMD R9 270x or GTX 950 would be a good budget fit for gaming in HD. You would still need to reduce some graphical effects, not only because of the GPU, but also so the CPU doesn't need to help calculate them.
http://www.pcworld.com/article/2973512/components-graphics/nvidia-geforce-gtx-950-review-bringing-more-oomph-to-budget-gaming-pcs.html?page=4

THX all for help