I think there's no such thing as "best time to buy hardware". For example: X-mas. U buy a video card, say GeForce 3, with some discount... a month later nVidia releases GF 4 and GF 3 prices go down. Bingo! U're a looser.
Or, Taiwan suffers an earthquake and ALL prices, especialy RAM, jet into the sky.
So, I think the best way to buy harware is to buy the "almost latest model" stuff. It'll give u near-top performance for reasonable price. This way u'll never-ever have the BEST OF THE BEST of... but u'll have a fine machine for a fair price.