The one with a Suntec branch would be Muthu's, famed for its Fish Head curry. But its gone upmarket, and so is not cheap. If you don't need a fancy place, try Banana Leaf Apollo at Little India Arcade(more spacious than the Race Course outlet not far away), it was actually started by the brother of the more famous Muthu's founder. Both these places started out as South Indian restaurants but have expanded their menu to include North Indian food.
But for good, affordable South Indian the best place is Gandhi Restaurant at Chander Road around the corner from Apollo. Its sort of a local's place that you won't find in any guidebook. Don't be scared off by the queue, that's only if you want to get takeaway. They don't have a fixed menu, you just choose what you want from what they have behind the counter. The chicken kurma and dalcha (curries) are the best, and my personal non-spicy favourite dish is the fish puttu(a sort of fried minced fish) and the chicken 65. My family gets takeaway from them once a week, and the rice they give you for a single set meal is more than enough to feed 4 people with leftovers.
Best is to go with a couple of friends, so you can get a few different set meals. Or if you can eat it all, just order additional dishes. As with most traditional Indian places, you get a free flow of rice, vegetables and rasam (a soup of sorts, but no bowls-you mix it into your rice).
Have fun!