Another thing to look at besides the sticky on making comparisons between the two kernels with the 64bit being a bit more efficient at memory management by less swapping out of data to the drive that is would be the particular system 7 will go on.
One example here is seeing XP drivers for the board, onboard sound, and old 128mb GeForce FX5200 video card all working on the temp 32bit 7 install. Obviously no Vista or 64bit support was ever seen for this oldie! On a much newer system readily having seen Vista 32/64 support as well as for the hardwares installed the 64bit will be seen as chewing about 300mb more of ram due to the reduction of swapping out to the drive by way of the paging file and seeing more data kept in the active ram instead.
MS then recommends 2.5gb as their "recommended minimum" for memory due to seeing this. That however doesn't specify 2.5gb is mandatory however but better suited since most systems now already see 3-4gb for most typical builders at least.