For HIDs, have to say I found them a very worthwhile investment, my lights before that were absolutely rubbish, even with the fancy philips whatever they were bulbs in. When I drove the car first when it wasn't completely dark (but getting there) I seriously asked myself if the lights were actually on!
Having said that, if you drive in mostly town (and thus lit) areas its not such an issue.
Brakes are always a good thing to improve too though... can't beat being able to stop quickly! Ms enzo, is yours a GS, or GR or GPX or GX or... ? Where I'm going with that is if you have a GS/GR you might be better off looking into upgrading to GPX front brake calipers (or better) rather than spending much on GS/GR discs and pads.