first of all i think you should buy dvp kit, its waste of time to design by yourself
what do you mean by JTAG OR USB ?
jtag work thru USB or Parallel port
i mean you will use JTAG anyway , the question should be USB or parallel
in this case i think if you want to design your own dvp board use parallel
it would be easier...
anyway in alera site search for BYTEBLASTER and you will see
a schematics of JTAG CABLE , so you'll be able just to duplicate it.....