Please, help to choose development kit based on Virtex-4

Hello All

I want to learn FPGA programming in VHDL using Virtex-4 chip from Xilinx in order to:

1. learn the trade really (I am a beginner and want to practice in FPGA programming on real chip).
2. develop my own system-on-chip which would be a part of experimental device (in physics)
controlled from PC host through USB interface (between Virtex-4 and PC host).

What development kit I should choose for my targets?
I was reading about many development kits from Xilinx which work with Virtex-4 but have difficulty
deciding which one to buy for my work.

Could you, please give an advice on this.