I would say now is as good as any. The Sandy Bridge dual Xeon platform was just released last summer so thats not going to change for a while.
And I would avoid the Quadro series of cards HP sells with them (get the cheapest Q2000) and swop in a GTX Titan for more power. And that was just released.
Do it thru the internet and you could get around 30% discount. Dont forget to get a good external raid if you will editing in an NLE as well as using AE.
Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
https://www.youtube.com/watch?v=DxPrG3WUyz8
(Best viewed at 1080P and full screen)
HP Z820 Dual 2687
64GB ram
Dulce DQg2 16TB raid