Win7 Pro on 64bit Vostro = 32 Bit Version?

My Vostro 200 came w/a 32 bit version of Vista.  Technically, the machine should handle a 64 bit OS... even MS's upgrade tool says this.  Intel's web site says the Core 2 Duo has this capablility.  But when I upgraded to Win7 Professional using the custom option (completely wiping the disk), it didn't give me a 32 or 64 bit option.  It just installed the 32 bit version.  Is there some kind of 32 bit limitation imposed by Dell on this machine?  Did I forget to say the magic words?