It appears wind 7 64 bit requires a min of 2 Ggs of ram, How much memory do you have?
And as pointed out above, if 4 or less gigs of ram there is NO benifit going with 64 Bit OS. If right at 4 gigs, yes you will gain about a 1/2 gig boost - But not really worth it.
64 Bit is only anvantagous if going above 4 gigs of ram.
How many 64 bit programs apps are you planing on running - NOT a whole lot out there.