I have done a lot of research on this. The best combination is Philips HID bulbs and Hella ballasts.
Hella ballasts are standard OEM on BMW, MBZ, Lexus, etc.
Philips HID bulbs are available in two types: D2S and D2R. The "S" stands for Shield, and the "R" stands for Reflector. D2S are used for cars with projectors housing with Shields. D2R are for non-projectors such as the new 350z.
OEM bulbs from Philips are rated at 4100K and 4300K color temperature. Philips also makes a 6000K bulb, but it is illegal to use in the US.
Color temperature at 4100K & 4300K are pure white with the highest output of lights. This is why they are standard OEM choice. Higher color temperature would give you a bit more color (blue, purple), but the light output decreases.
Some people buy HID retrofit kits such as K2, Eagle Eyes, or Misterjungs thinking they just pop it into the Celica's housing and they would be just as bright and pretty as the BWM's or MBZ's. Or they buy the higher temperature kits such as 8000K thinking that it would give them the S2000 look.
The truth is that everything has to do with the light housing. The S2000 uses the 4300K bulbs, which shines pure white if you look straight into it. The pretty prism effect (blue/purple) that you see when a S2000 comes down the street is caused by the optical design of the projector, and not the color of the bulbs.
The Celica has free-form projectors. Unfortunately, it does not have shields (light shines everywhere). Fortunately, the Philips D2R bulbs fit perfectly in the H7 slot. How do I know? I have the HID kit from an Infinity G35 with Philips D2R on my car. The D2R bulb has a built-in shield which shields off the sides and the bottom-half of the bulb. This reduces the glare and gives it a much sharper cut-off. And it eliminates the shadows caused by the return wires.
If you want your Celica to look like the S2000 or BMW, the best thing to do is to replace the stock projectors. I found that the projectors from an Audi TT (very small) would fit nicely in our Celicas. These projectors are designed for HID's, and would give you the pretty prism effect. Of course, you would have to take your headlights apart to do this. A complete HID kit with Audi TT projectors, Philips HID bulbs, and Hella Ballast would run about $450. There are companies out there that would do the installation, including HID kit for $800 - $1000. You would have to ship your headlights to them. The whole process would take 3-4 weeks.
I started off with the Mcculloch 5300K retrofit kit. It was bright, but the lights were everywhere (lack of shield in our projector). So, I replaced it with the G35 kit. And I am very happy with it. But, it is still not as pretty as the S2000 because our projectors weren't designed for HID's.
Hope this helps. Damn, this has got to be my longest post.