Try reading
this first so that you know what you are getting into.
You can get by with aircooling but you need sufficient crossflow of air thru the case to be effective. Heatpiped HSFs are often an effective aircooling system, the Big Typhoon, Freezer Pro, and CM GeminII are good examples. Also note that ram, the voltage regulators, chipset and vidcard may impose additional cooling requirements during overclocking, then there are the HDs that run hot. There are watercooling kits that cool the chipset and vidcard, someone also recently released heatpipe coolers for ram. Note that watercooling also imposes its own set of problems such as potential leakage and algal infestation. Additional fans also mean a higher noise environment, you might have to look for high cfm, low rpm fams to abate the fan noise. If you think fan noise is a non-issue, try living with a Volcano 6+ for a year.
Aluminum cases are not exactly necessary if you have good internal airflow, though the heat transference of aluminum is quite good. More often than not, ambient temps play an important role in cooling overclocked systems since even watercooling still relies on air to cool the radiator. An additional note is to replace your power supply with a more powerful one when OCing; generic psu's will just not do.
