Would a professional GPU or gaming GPU be better for light gaming?
I'll list the key differences between professional and gaming GPUs so you ccan better understand what you need and why:
- Professional GPUs are much more expensive
- They come with error-checking and correcting VRAM (slower, more relieable even though faults are rare)
- They come with special drivers which have special boosts for certain creative programs
- They support 10-bit color output via OpenGL
As you don't care about the last three points, it is a gaming GPU for you.
Or do I even need a GPU?
You have picked two Intel Xeon E5s. These do not come with a graphics chip on the die, so you will need a dedicated GPU if you don't want atrocious graphics performance, which will be really annoying even for light desktop activity.
Do "retro" games rely more on CPU or GPU power.
I'd say they rely less on the GPU than modern games, however you don't need to worry about the CPU with your current configuration.
I've looked at the games and it seems like you need a graphics card that has like 1GB of VRAM to be good in all of them. Depending on the offers you can find, something like a 750, a 950 or a 1050 should all be able to do the job well, these cards are about 100 USD on amazon, so you can probably find them a little bit cheaper elsewhere.