the astra's reflector type headlamps were not designed to be used with HID globes, as rjastra pointed out. its focal point is different. and if u do choose to anyway, make sure u get the shielded hid globe type (D2R). u'll still get light scatter.
u need to get projector type headlamps, and as cnblu said - angel eyes r one example.
however, not all angel eyes, or projector headlamps can handle hid globes. this depends on what material is used for the projector lens (not the headlight cover). cheaper ones use polycarbonate cause its cheaper to manufacture. however these will discolor overtime due to the heat produced inside by the hid globe. eventually no light will emit through.
the same thing will happen if u retro fit hids into ur stock headlights. the headlight cover is made of the same polycarbonate material. u'll cook it from the inside, n eventually discolor it.
the more expesive ones use glass, n hence can handle the heat produced by the hid globes. they will also shield the headlight cover from the heat.
oh...n i wouldn't recommend anything higher than 6000k. unless ur going purely for looks.