Well, if you really want to absolutely wait, maybe for Vista only. Other than that, tech. news and improvements are happening every single day so once something new comes out, it'll get pushed over as a newer tech. comes out the following weeks or so.
However, if I was you, I'd start building right now. If you want to build a Vista PC, you already know it's specs and what it needs, so you can just build a fully capable Vista machine and install XP or an OS on it and wait. Other than the other things such as hardware wise, you can if you want, but it's not like it'll make your pc the best in the field, it'll get outdated pretty soon.