The South African midfielder's first goal of the season arrived with six minutes remaining as the Eagles finally found a way past Bo Myhill in the Birmingham goal.
The Blues arrived in south London looking to kick-start their Championship promotion bid after they bowed out of the Europa League last week - they kicked off six points shy of the play-offs and with three games in hand on the rest of the division.
Myhill looked to have ensured that gap would be cut to five with a string of fine saves, while the woodwork denied Eagles skipper Paddy McCarthy.
But Palace were not to be denied and the man known as 'KG' conjured up a dramatic late winner for Dougie Freedman's side.
Palace announced before the game they had tied their latest Academy starlet, Wilfried Zaha, to a bumper new five-and-a-half year contract as they bid to keep a host of top-flight clubs at bay.
Tonight's match could have done with some of Zaha's pace, skill and eye for goal but, sadly, the 19-year-old winger was suspended.
His replacement, Darren Ambrose, attempted to recreate his Carling Cup heroics at Manchester United last month but this time his 25-yard shot sailed high and wide.
The game began to open up after a cagey start and Birmingham striker Marlon King volleyed Stephen Carr's cross over before Eagles keeper Julian Speroni made a meal of turning a low shot from teenage winger Nathan Redmond wide.
And when Chris Burke swung in the ensuing corner Curtis Davies, lunging in at the far post, could only toe Guirane N'Daw's flick-on over the bar.
At the other end Myhill was at full stretch to tip away Mile Jedinak's sweetly-struck volley, and full-back Nathaniel Clyne's charge into the area was only halted by a superbly timed, last-ditch tackle from David Murphy
Jedinak, who has yet to score for Palace, had another chance when he met Ambrose's free-kick with a firm header but Myhill dived to his right to make another superb save.
And in the 69th minute Eagles skipper McCarthy arrived at the far post to meet Ambrose's corner but his header crashed back off the foot of the upright.
Birmingham could have nicked a goal as full-time approached when Speroni parried Burke's angled drive but Caldwell headed the follow-up over the top.
And instead Dikgacoi had the final say after the visitors failed to clear Ambrose's corner.
Palace fed the ball back out to Ambrose on the right and when he swung in the cross Palace's giant midfielder powered his header past Myhill.
