Thaddeaus

Quad Channel FB-DDR2, the memory bandwidth wasnt a problem, I have/had a DDR3 C2Q to compare against.  The general ineptitude of Croe 2 Duo/Quad generation cores was the problem.  Per-core and per-clock they are 20% behind 1st gen Core CPUs (i3/i5/i7), and their clocks are generally lower.  Compared to Sandy Bridge, they are 50% slower at the same clock.
 
In games like Fallout 4, FPS swings wildly in outdoor areas... swings from 20 to 80 and back are pretty common.  In GTA 5 you can expect 20-40 FPS unless you set lots of things to low.  In both cases the GPU is generally capable of a steady 60 FPS if the CPU wasnt in the way.

Some games won't run at all. Like apex legends, because lga 771/775 does not have the SSE4.2 instructions.
 
Also the real bottleneck is the ddr2 ram. It's usually running at 667mhz. If you have a board that supports ddr3 there will be very little bottleneck. It's almost as powerful as a first gen I5. 

Yes, it will.  Up to 50% in some games.
 
I had two of those CPUs in a dual socket with that exact GPU.

Both fairly low end devices, I wouldn't concern yourself with a bottleneck.

Mechwarrior Online, Elite: Dangerous in some environments with ultra details, Elder Scrolls Online, etc.  Mostly games from the horrid days of multithreaded programming.

What games is it having issues with?

I have such a setup, it struggles with keeping GPU usage up on an RX 470 on some games, in some cases only pushing 30-60 FPS.
 
R5-1600 has issues with the same games due to it being roughly the same on a per core basis.  It has better per clock performance, but lower clocks.

an overclocked i7 2600K should be able to do 60-100hz without an issue.
 
But I'd probably just go for a really cheap R5 1600 or R5 2600

Correct.  Just crank SSAA or something similar, instantly GPU bottlenecked again.
 
Get the best GPU you can afford in your budget, the CPU/MB should be upgraded soonish though.  2nd gen i7 + H61 board will get you a platform that can push up to a 1060 without much struggle.

The RX 460 is a lot faster
 
that's not really how bottlenecking works
 
if your CPU is stuck at 30-60 fps in a given game, you can just turn up your GPU settings so the game still looks better on the RX 460.

Xeon X5450 is a 4 core, non hyperthreaded Harpertown (Yorkfield) which is around the Core 2 Quad era of consumer processors, and is clocked at 3ghz.
 
I do not believe that it is overclockable, and I also believe that no matter what GPU you pair this with, the performance is going to be pretty bad, and neither one will be fully utilized.
 
 

You replaced a 65W cpu with a 120W CPU, even your motherboard might be having issues with it. On top of that you got a 170W PSU for all that. Everything is possible in your case.

Try a better power supply to see if it boots properly