matty, as above the h4 dual element kits are what willbe needed for your 33, and the best versions are probably the hi/lo kits, which as above, have the moving reflector back and forth along a single xenon lamp.
if it is a decent kit, and installed carefully, you stand a good chance of getting a vastly improved dip beam, a useful gain high beam, and both with reasonable pattern and cut off.
the piggy back h4 kits are really only any good for dip beam improvements, but the likelihood of getting an accurate beam cut off/pattern is slightly less on the dip, and next to impossible for the main/hi, and this is because the piggy back halogen section of the bulb, is simply to prove you have a hi beam option, fitted, and illuminating, in other words, prove to a roadside copper that you have something in there, that lights up when you flick the switch, but these will stand little to no chance of conforming to any legislation if actually tested.
the single lamp conversions should work well, because as above again, the actual headlamp unit takes care of the cut off/pattern, from pretty much any source of light, the lamp itself is not designed as a focal component, so you should have all the gain, and far less chance of stray light and warnings of blinding folk.
but, as above recomends, make all this type of retrofit kit, as easily reversible as possible, which is actually quite simple, because the bulb specific kits normally have a section of loom that connects to your original headlamp feeds, so, to reverse back to the original halogen lamps, should be a case of.....
remove the Xenon lamps, and refit the halogen lamps into the headlamp unit,
disconnect the headlamp feed connection to the HID kit, and replug it onto the halogen headlamps, job done, your back to OE.
we will all need to do this, because the MOT system is stating the following, and should have become legislation this years first quarter,
"all cars fitted with aftermarket HID/Xenon conversion kits, must have auto adjust (headlamp aim) and headlamp wash systems, fitted and working"
so how many of us are going to fit them to our ageing beauties?
this is clearly aimed at destroying the aftermarkert HID kit availability to the general public for DIY install, and because, it is very easy to buy a kit, diy fit it, possibly very badly, and this is the reason for complaint regarding being blinded by the oncoming Vauxhall Corsa Bad Boy who's beam pattern is none existent, his lamps aren't even seated correctly, and has more chance of spotting planes than seeing the road ahead!!
now we must all suffer......
it can be done, with a cheap kit, and very successfully, as i and many others can verify, but it also easy to get wrong.