For $800 you can buy a 2500K and 500-series yes, but probably not a new screen. At least not a quality screen. I recommend you use $600 on the computer (not enough for 2500K), and $200 for a new screen.
Ok i use my old pc screen
But for $800 with 2500k and gtx 5xx I still some money left? if yes what do you recommend for ram,psu,MB and other?