hey funkey_munkey.... You're not ian are you??
Dual core: basically just what you said, two cores located on one CPU.
HT: This is a virtual dual CPU setup. So your computer thinks there are two CPU's in your system. Usually better performance when running more than one application at a time.
64bit: Only required if you have a 64bit operating system such as the new 64bit Windows XP or Linux. This also increases performance as the system can compute larger numbers.
Neither of these are of much benefit to the average user who surfs the net and uses MS office.
However, for multimedia use the can considerably increase performance, especially if you specific 64bit software to use on your 64bit OS. For gaming the dual core and HT features can increase performance. (notice i use the word 'can' and not 'will')
However, the general trend is gearing towards these features anyway, so if you buy new system chances are you'll most likely have a HT and possilbly dual core processor in them.