With an OEM version you are allowed 3 hardware changes IIRC (as certain hardware gets logged into an MS activation database).
So, if you change quite a few components you are going to trigger the reactivation console. It?s happened to me quite a few times and I have to ring up to activate it.
I think you should be ok with a retail version, as the license permits you to install the copy on any machine so long as only one machine uses the license.
With an OEM copy, the license only allows you one installation with the original hardware. This prevents you from installing the same copy onto a new PC, and tries to promote you purchasing a new license. It?s a bit pointless as I?ve installed my OEM copy of windows XP onto several new configs over the years now, and MS don?t seem to care.