Dynamic Czech midfielder Pavel Nedved gave the reigning champion an early lead before David Trezeguet scored twice to take his tally for the season to 13.
Fabio Vignati completed the Sardinian visitors' misery by scoring an own goal as Juve claimed its 14th league victory in 15 matches.
Meanwhile, Brazilian hitman Adriano scored twice for Inter Milan in a surprise 3-2 derby win over city rival AC Milan.
Juve's advantage leaves it well placed to retain the Scudetto but coach Fabio Capello insisted there was still a long way to go.
"There are many matches to play and many points at stake," he said.
"We have to keep on going with the same determination and concentration."
Adriano's 91st-minute header condemned AC Milan to its second successive league defeat.
His penalty was canceled out by an equally dubious spot-kick that Andriy Shevchenko converted before Nigerian Obafemi Martins capitalized on a goalkeeping error by Nelson Dida to put Inter ahead again close to the hour mark.
Jaap Stam appeared to have salvaged a point for AC when he headed home Andrea Pirlo's free-kick seven minutes from time, but Adriano, nicknamed "The Emperor," snatched a dramatic victory by heading in Argentine Juan Sebastian Veron's corner.
Livorno strengthened its grip on fifth place with a 2-1 home win over Lazio.
Diego De Ascentis put the host in front before Macedonian striker Goran Pandev leveled the score.
Livorno secured all three points 12 minutes from time when Cristiano Lucarelli's half-volley took a deflection on to the post and then ricocheted into the net off the chest of Lazio keeper Angelo Peruzzi.
Lazio striker Paolo Di Canio could find himself in hot water after once again giving a fascist salute while he was being substituted. Fixtures between the two teams are always tense affairs as politically Livorno fans have left-wing leanings, while Lazio supporters have links to the far right.
Messina eased its relegation fears with a 2-0 home victory against Chievo, while Lecce moved off the bottom after its 2-1 win at Udinese where Serbian striker Mirko Vicinic grabbed both goals.
Roma slumped to its fourth home defeat, a 2-1 loss to Palermo, while Ascoli against Reggina and Parma against Sampdoria both ended in 1-1 draws.
In Turin, Patrick Vieira unlocked the Cagliari defense with a sublime pass to help put Juve ahead in the 10th minute
The Frenchman's exquisite chip picked out Trezeguet whose close range shot was palmed away by Andrea Campagnolo, but Nedved was on hand to collect the loose ball and smash it into the net from six yards.
Cagliari fell further behind seven minutes later. Zlatan Ibrahimovic's flick was headed on by Adrian Mutu and Trezeguet toe-poked the ball in at full stretch from the tighest of angles.
Trezeguet scored his second goal shortly after half time, firing a low drive into the bottom corner after a sweeping move involving Nedved and Ibrahimovic. The France international then played a major part in Juve's fourth goal in the 68th minute, his angled shot coming back off the post and bouncing off Vignati's shin before crossing the line.