Noob question, simple answer
 

If your car's headlights have been converted to H4, can you still use regular halogen bulbs in them?

These are my headlights that were installed by the previous owner, IKI H4 headlights w/ a conversion kit.
http://img409.imageshack.us/img409/4...x768kh1.th.jpghttp://img341.imageshack.us/img341/3...x768ar0.th.jpg

I'm thinking of installing Sylvania Silverstar H6054ST. What do you think (I hope they're brighter!)?

As long as they arent an HID kit, then yes should be no problem fitting whichever H4 bulb you want in there.

But the Sylvania Silverstars aren't H4, are they? I know the H6054 would fit a stock FC, but mine has undergone an H4 conversion (not even quite sure what changes that means were made).

Most of the H4 conversion kits are simply a different headlight assembly that is retrofitted with a hole to accept an H4 bulb. The stock headlight bulb was a sealed unit meaning you would replace the entire headlight assy lens part. This seems like its your average H4 conversion piece that will accept any H4 bulb. Sylavania Silverstar H4 bulbs are only sold in single packs and are for "motorcycle use only". I however have used them in my DD protege and love em. Gonna get a set for the 7 when I get around to it.