As the ArticSilver cures, your temp will continue to drop a few degrees over about 3-5 days, especially under load. One solution to a hot case is a duct for the CPU to the case side, which intakes cool air.
High 50's isn't too bad for an overclocked Athlon under load, my OC-XP2600 runs about 53' under load with AS3 and a copper heat sink.
Highly recommend Panaflo fans for case cooling, they're the quietest available for the CFM ratings. "Panaflo medium" is good for most case applications, Panaflol High is pretty robust. Just be sure you wind up with similar CFM ratings on the intake and exhaust fans. I have 7 case fans running here. :o
Also, take into account what you are using for temp sensing. Motherboard diodes are notoriously inaccurate. There's no substitute for mechanical sensors, especially for case temp and CPU temps when you have a heat problem.