PC Gaming Parts Help

I was thinking of building a (budget) gaming PC but i'm really at a loss as to which parts to choose, specially GPU, CPU, SSD (v HDD) and the case. I considered pairing an I3-8100 with an RX 580 8GB but i dont know if i could get a similar (or better) perfomance at around the same (or lower)...

Hey guys, thanks for taking the time to view my thread. First things first, the setup: i5 3570k @4.2 970 x2 SLI 16gb ram 1800mhz (i think) Windows 7 64 hp So a while ago I would get 300-500fps in csgo and now I'm only getting 130-280 I know the game updates and can cause fps drops but surely...