David LR does not need a lot of resource
You say the Motherboard will accept an i CPU, the socket for the Intel G2020 is (1155) I may be wrong and often am and you already have checked this socket fitting, ignore if so.
Throwing an expensive i7 or even i5 and the CUP is where the money needs to go, braver man than myself to put it in an old motherboard, hit and miss if the motherboard would use anywhere near the potential of the CPU, if it worked at all. I'd find out for certain before hand. Even if the fittings were compatible, the BIOS system may need to be upgraded which on an old board is risky if at all possible, thankfully new boards this can be flashed, yet this still carries risk.
Compatibility of parts is a must and lower specs of RAM and CPU and motherboard which are a marriage made in heaven will always give you improved results over the most powerful parts which do not run seamlessly together which was always the problem in the past. Thankfully today we can harvest enough info online to resolve these compatibility issues.
Yes agree fully a dedicated graphics card is a must for an older board. LR can use your Graphic card as an extra resource, a tick in the performance box in preferences within adobe software is all that is needed.
SSD lubberly, seconds to boot, only problem on Windows 7 8.1 and 10 is SSD can be ejected by mistake. A quirk and seen as an external drive. Need to be a decent size if using it for a scratch disk as well, although it will, the larger the better.
and yes only have the OS with programs on this and one or more internal hard drives for files.
I got an intel CPU only by recommendation from someone I know who is experienced. It is the hub of everything and the greater the cores is far better than loading with Ram. Many used to fill their 32bit pcs with Ram and of course 32bit could never utilies it . I have 16GB and thought about the 64GB the motherboard can accept, exponentially I'd recon that I'd not see a real world difference over 32GB and decent Ram is dam expensive, also everything works very well. I am sure you know this, ignore if so. Always buy RAM in a set, they test each stick set together to optimize compatibility. Random sticks often wont function as well as the sets. Apparently.
So yes I have an Intel CPU and it runs Sigma Photo Pro, Cs6, LR, and umpteen other programs on screen drawing memory and CPU resource seamlessly, no issues with what ever.
I got fed up of of the shelf computers and built one. On reflection I'd have got the next higher CPU but that would have been greedy really. I did have lots of old parts like you have, given to me for free, but when looking into it and Sigma software can be slowish on any system for some, I decided to get parts that would last for many years hopefully. I chose a Work station motherboard that are designed to have a reliable data management system running a constant 24/7 . Also very up gradable, will take 64GM RAM and has 6 PCI sockets for a series of graphic cards or whatever takes your fancy. I could not have got this spec of the shelf at the time as it was purpose built for my own imaging needs and the software I have. I could have got an online company to build the same specs. It cost me a bit less than two thirds so they would have charged nearly thirty percent more, it took about four hours tops to assemble and boot it up with Win 7 at the time. I was a novice learn t a lot at the time and enjoyed the experience and have forgotten lots. Upgraded to the free win 10 while it has been free.
Sorry written far to much probably about nothing, but if one little thing helps I hope it does.
This is my simple system
System Type x64-based PC Win 10
ASUS P9X79-E WS. Lovely work station MB.
Processor Intel(R) Core(TM) i7-4820K CPU @ 3.70GHz, 3701 Mhz, 4 Core(s), 8 Logical Processor(s)
Installed Physical Memory (RAM) 16.0 GB GDDR5
Intel R9 270
Samsung 850 EVO SSD 500 GB