Try - expensive: (£12-18 for a main)
Deanes Brasserie on Howard Street
Roscoff on Bedford Street
James Street South on James Street South
Cayenne on Great Victoria Street (my favourite)
Tedfords Fish restaurant overlooking the river off Musgrave street (my favourite)

Medium: (£9-£12 for a main)
Ginger (great fish) on Hope Street
Deanes Deli on Bedford Street
Greens Pizza on Lisburn Road (small family run firm - best pizzas)

Also found a great bar on Skipper Street called the Spaniard - it is a wonderful tiny bar in a sort of Spanish style. Really cool.