So it's a Pentium 4 651 3.4GHz? What motherboard you have? It should be powerful enough for WinXP retro gaming (2000-2006 games) even at stock, for newer WinXP games you should use some Core2 Duo.
With a GTX 750 Ti overclocking the CPU probably wouldn't make much difference if at all. But it's still a pretty decent budget card for some lighter games etc.
For music and movies I'd just keep it at stock. Best program to use it as HTPC for movies etc. would be Kodi.
With a X5650 you should be able to hit 4GHz easily even with some cheap cooler (but not stock cooler).
X5650 @ 4GHz should beat the i5-4570 in most games, if you use some better graphics card than the GTX 650.
I'd get the 1440p monitor. Supersampling 1440p on a 1080p monitor to get more real estate looks pretty bad. And your graphics card should be able to run most games at 1440p
What is your monitor resolution and refresh rate? It should be fine with the i7-2600K. Of course you wouldn't get as high fps as with i7-8700K, but I'm sure you will get a lot higher fps than with the GTX 770.
For gaming I'd probably go with the ultrawide. 29" ultrawide is as tall as 23" 16:9 monitor so 1080p is just fine.
For anything else I'd go with the 1440p.
You probably need to set Ai Overclock Tuner to MANUAL to be able to increase the bclk. I've only bclk overclocked a Xeon X5670 on my P6X58D-E so I'm not sure how different it's from those unlocked i7s.
Currently I have my X5670 overclocked to 4.32GHz with 1.35V.
I'd recommend checking this thread:
You are upgrading from XP to 10? What is your PC specs? Many older Windows XP PC's can't run Windows 10.
Like others have already said, you can use Windows 10 without activating it.