A draw against Charlton on Tuesday will be enough to secure the Welsh club's return to the top flight after a 51-year absence.
Helguson's ninth goal of the season had given Cardiff the lead before Darius Henderson was sent off for elbowing the veteran striker, and Gestede then sealed the win with his second-half brace.
Henderson had missed Forest's best opportunity when he failed to convert from close range, while Lewis McGugan had an effort ruled out for a foul on Aron Gunnarsson as they dropped out of the play-off places and their 10-match unbeaten run ended.
Watford's defeat at Peterborough means Championship leaders Cardiff are now 12 points clear of the third-placed Hornets with four games remaining.
The home side who took the lead in the 26th minute. Kim Bo-Kyung won a free-kick after fooling Greg Halford, and the defender then lost Helguson from Bellamy's free-kick for the striker to nod home at the far post.
McGugan thought he had levelled the scores just three minutes later when he got the better of Gunnarsson and curled home, but referee Andre Marriner ruled he had fouled the Iceland international.
Marriner then turned down Cardiff penalty appeals after Guedioura appeared to foul Jordan Mutch as the midfielder turned his marker.
The official soon had to cool tempers when Marshall and Henderson exchanged words after the striker stayed down after a corner, with the dispute spreading to the touchline as tempers frayed between Malky Mackay and Billy Davies.
Henderson had clearly not cooled off when he was dismissed with just three minutes off the first half remaining, receiving a straight red for a blatant elbow on Helguson.
Things got worse for Forest when Halford was carried off two minutes after the resumption, having been caught in the face by Gestede's leg as he attempted a clearing header.
Cardiff were by now firmly in control and their second arrived on the hour.
Guedioura and Dexter Blackstock were booked for cynically halting a Cardiff attack and Bellamy curled the resulting free-kick against the post.
But the danger was not over, the ball fell to Andrew Taylor and Gestede was on hand to convert his cross from close range.
Six minutes later and it was 3-0, Bellamy supplied the corner and Gestede was once again made no mistake with his header to delight the record crowd at Cardiff City Stadium.
Karl Darlow in the Forest goal making a wonderful save after Kim's silky dribble, before Bellamy hit the outside of the post with the goal gaping from Gunnarsson's ball across the box as Cardiff cruised to victory.