Seriously, it is best to buy most parts close to the time you will use them. Technology prices continuously drop. Supply and demand still rules.
Planning ahead, though, be on the lookout for deals on cases. Cases do not change much. PSU's also. Antec often has good sales and rebates. Couple that with free delivery and you can't go wrong buying early.
High end graphics cards will be expensive for early adopters. Wait two months on those if you can.
SSD prices and technology are changing by the week. Intel will sample 25nm chips next quarter. By the end of the year, expect price/performance to improve greatly. My two month rule will apply on them for the next two years.