Whatever you do, do not buy a Harbor Freight welder. They pretty junky and quite unsafe in some ways.
I've been looking at this Lincoln welder on eBay
http://cgi.ebay.com/LINCOLN-SP135T-M...cmdZViewIt em
and this aluminum kit for it
http://cgi.ebay.com/LINCOLN-K664-2-A...cmdZView Item
This seems to be the cheapest way to go. For how well it'll work, you can bet it'll be better than a Harbor Freight, but I don't know just how good it will be. It seems to be a pretty nice little setup. For doing aluminum, I've never tried but everyone says you should do it with a TIG, but it can be done with a MIG. As for actual experiences with the MIG and aluminum, I haven't heard much beyond "it can be done"
I've also seen rods made for stick/arc welding aluminum, but I've never tried it and haven't heard anything about them. The same goes for oxy-acetylene welding aluminum.
This may be obvious but try to get the highest current welder you can with the money you have. You will always have a welding job that could use more current, especially when it's aluminum.