Cleaning: Welding of aluminum is made difficult because of how rapidly it can form an oxide layer on its surface. While this is what makes it so resistant to corrosion, this layer must be removed prior to welding. This can occur in a number of ways, either by mechanical cleaning like a wirebrush or abrasive pad; chemical cleaning using a variety of industrial solvents or electrically by the bombardment of ions.
Stainless steel wirebrushes are a good choice, as they are both very effective and cheap. Care should be taken to prevent contamination of the brush (either from oil, dirt or rust), which could impart these contaminates onto the material and into the weld. Powered wire wheels should be avoided as these have a tendency to smear the material and further trap contaminates instead of removing them. Steel brushes should be avoided if possible because they can leave behind iron oxides (rust) on the material.
Wiping with a solvent such as acetone, denatured alcohol, or others is a good idea, but make sure that all solvent has been evaporated or removed from the weld joint before an arc is struck to avoid the possibility of dangerous fumes or fire. However these solvents will not remove the oxide layer, only excess oil or dirt.
Abrasive mediums like 3M pads, sanding or grinding wheels or stripper wheels work well for removing heavy contamination like is often found on castings or parts that have been in service. These will remove surface oxides as well, but can also smear the material if to much pressure is applied.
How clean is good enough? I would be lying if I said Ive never successfully welded material that had heavy oxides on the surface, or had some leftover dirt that got pulled into the weld. Ultimately the decision is yours to make, but keep in mind that the cleaner the material, the easier it will be to weld, lower chance that cracks will form and overall leads to a better quality weld.