Solid state HDs would be the first thing, for me. I've got a pretty decent kit right now, and don't think I could justify upgrading till at least 2007, if not later. Unless, of course, something revolutionary happens. Solid state HDs being one of them.
I'd never cough up money for a BD-ROM drive (or HD-DVD, for that matter), nor the silly physics chip. Unless Ageia somehow manages to push their chips at cut-rate prices, no-go. Sure, some games will take advantage of them, but not in any significant way. Even if they get 10% market penetration in a year, that's 90% that don't have it. Game developers cannot optimize a game for such small market-share. They can do little things with it, but not actually base the engine on having dedicated hardware.