Trevor Bayliss' side lead 2-0 as they prepare for the third game of the five-match series at Centurion on Wednesday.
Victory would see the one-day team emulate their Test counterparts, who beat the Proteas 2-1.
"We said this would be our hardest challenge in the last year and it has been, but brilliant performances have led us to where we are," said Topley.
"To follow the Test boys and get two series wins out of two, that would be great for English cricket."
Hampshire's Topley, 21, took four wickets as England went 2-0 up with a five-wicket victory at Port Elizabeth on Saturday.
South Africa batsman Hashim Amla says his side are treating the rest of the series as three finals in a row.
Amla, who resigned from the Test captaincy during the middle of the series to concentrate on his batting, said: "Everything starts on Tuesday.
"We're out to win, get some momentum and then hope to win in Cape Town as well."
England winning their one-day series in South Africa would "mean everything" to the team, says bowler Reece Topley.