Regret, sorry this thread is going a little off-topic, it happens.
Honestly if you want to buy piece-meal to December I would only focus on components that don't have a tendency to price drop very quickly.
-Case
-Keyboard
-Mouse
-Optical Drive
-Windows
Might be okay to snag the motherboard, RAM, and power supply, though if you wait to get a video card you may find the best you can afford in 4.5 months requires more power than the power supply you bought today.
In 4.5 months I am willing to bet the price of the other components will drop significantly.
Intel has a new series coming out Q1/Q2 of 2012, prices on the i5/i7 should be dropping.
New video cards are always under development, older ones are always dropping.
Solid State Drive technology is improving rapidly and cost is decreasing.