if youre looking for a hotel that is decent priced and smack bang in the middle of the city, try the pacific in hindmarsh square.
as for nightlife, i myself like the smaller underground clubs & bars - i reccomend Sugar on Rundle St. and even Dragonfly in Victoria Sq. on a Saturday night. Theres heaps of other joints that are popular - Electric Circus, Garage, Dog & Duck, Vodka Bar, or even HQ like Batmaan Bin Suparman said above (lol). And while you're down here, you might as well check out the worst Casino in Australia...