I had a Xantia a while back - both main and dip (seperate) bulbs are in the same headlight housing. They have a big problem in that the dip cut off is quite low, so if you adjust the lamp up, the main beam shines at trees, not the road.
I fitted a set of Phillips high intensity 55W bulbs, and to my surprise they really improved things. Just done a quick google, and found these
http://gbdriver.co.uk/index.php?main_pa ... 1f727c9535. can't be sure they are the same thing, but looks like it. Pricey though.
As for the HID conversions, there has been a lot of discussion on here previously about them. I have a set in a Toyta hilux, and I can confirm that they are brighter. Much brighter. However, there are issues.
1. Sone HID conversions are dip beam only, so you end up with no main beam. Obviously not 100% legal.
2. The construction & use regs for HID bulbs stated that the lamps must be self levelling if fitted with HID bulbs, and they must have a lens cleaning system (to prevent / reduce glare from the higher intensity lamp). All the manufacturer's standard fit HIDs are compliant with this. Bongos, obviously are not (neither are Hiluxes!) - not legal, but a fine point which may or may not be known by a policeman doing a roadside check.
3. Biggest issue really. Fillament bulbs have a very small emitter (the fillament), about 5-8mm long. So, the reflectors and lenses are designed to work with this light source which is almost a single point of light. The HID bulbs have very long emitters, about 25-35mm long. There is no way that the lamps will perform correctly with these if they are designed for a filament bulb. in theory, you will have a crap beam pattern, and lots of wasted light going where you don't need it and not enough light on the road. In theory, the dip beam cut-off should fail an MOT.
However, in practice the Hilux does have a lot more light 'down the road' with the HID bulbs. I have driven with it following me, and the dip beam cut-off is fine.
I guess it's a case of paying your money, taking your chances that a) they will be better, and b) you won't get pulled over by a smart-arse copper who's been reading up about the legality of retro-fitting HIDs to older cars. As this becomes popular, I imagine that there will be a number of such officers of the law.
As for my Bongo - the lighting performance out the front seems OK (no idea what bulbs are fitted, still whatever came from Japan). But I might try HIDs anyway one day when I've run out of other mods to do.
BTW, the ballast is just a wee circuit board in a waterproof box that takes the 12v feed from the car wiring and amplifies it to a very high voltage to create the arc required to light the HID. Once the arc is struck, the ballast controls the voltage to the bulb, typically 85 or 42 volts, a.c.