Assuming windows 7 follows the same pattern as vista, it'll work like this:
Retail Full and Upgrade boxed copies (home premium or better) come with both x86 and x64 discs in the same box, with the retail licence code working on either. They can be installed on any number of machines, as long as it's only on one at a time (too many reinstalls close together, and you'll need to do phone activation though)
Upgrade copies using upgrade keys need a qualifying install on the hard-drive before they will proceed to the select partition stage; you can format and install clean into the 'qualifying' partition, or create a new partition alongside it and install there.
Installing the upgrade disc using the 30-day trial licence, then reinstalling the upgrade copy again clean using the upgrade key at install (format and over the top) works.
OEM copies are x86 or x64 only; the free upgrade copies provided as part of the last 'buy a machine early, get a free copy of vista later' were oem versions equivalent to what shipped with the machine; so if you get a machine with OEM vista x86 home premium, you'll likely get an OEM copy of 7 x86 home premium only. This can only be installed on one machine, according to the licence - upgrade motherboard or buy new machine, and you need a new copy, though you can often talk phone activation into it if it's a necessary repair.
From what I can tell, the discount pre-order process in the US is for upgrade retail copies; i.e. both x86 and x64 in the box (or downloaded), but you'll need xp or vista on the hdd, or do a double-install to get a clean install session going with the upgrade key.
The discount pre-order EU versions are full retail windows 7 E, i.e. without IE, Microsoft have said they're not doing upgrade versions at all of the E versions in the EU. This is presumably why the UK pre-order discount price is much higher than that of the US (£49/$80 for home premium, £99/$164 for professional) as it's a full retail version, not the upgrade copy.