My little brother is wanting to begin is adventure into pc gaming so I've been helping him out with old parts I've replaced over the years to keep his budget down. I've got a dual set of nvidia 550 TIs or a Radeon R7 370. Not sure if he would be better off running the 550s SLI or just to run the single R7 370.

your probably better with the R7 370

It's not really beneficial to SLI or Crossfire imo

I have two AMD GPUs in my rig and don't crossfire ( both coming out soon though I need to upgrade again this new display is a beast

Give him the 370..if hes just getting into it, it's more than fine

r7 370 much better

What CPU and PSU is he using?
RX580's are going for as low as 80 bucks on ebay and it would be a major upgrade for an older CPU