Win7 seems to be last in between windows between 32bit and 64bit world. Win 8 seems to be first pure64 bit windows. I am guite sure that some companies will stick on vista or win7 for some time, while those who don't need old 32 support will move on to win8 after sp1 or sp2... So it's guite ok to buy 64bit win7 now and upgrade to win8 in 2013 after sp1 or 2... It seems to be so that win8 will be the core of for 6 years, but face uplift after 3 years (win9) like vista was suposed to be, and win 7 just an face uplift...
Win7 is most likely better compatible to "old" 32 bit programs than win8, so it may be guite popular quite long time after win8 is released, but who knows. Maybe 32bit support is so important, that win8 has buid in compability support from the beginning?