Incidentally there are a number of browser issues in IE8 BETA. If you can install Build 7022 (only 32 Bit available currently) then this has IE8 RC1 installed which fixes a lot of problems and for this reason alone its worth installing this build.
Unless you have (or are likely to install) more than 4GB RAM then you don't lose anything by installing the 32 bit OS. It's going to be at least a year to 18 months before we'll start seeing a lot more genuine 64 bit applications.
Cheers
jimbo