Need to update my knowledge

[#1]

Its been a loooong while since I built my PC, now it is time for a fundamental update. I will build completely new but sorry for a few basic questions so I do not go down the wrong learning curve
I understand about being able to access more memory with a 64 bit OS, but what are the dissadvantages?
Do applications have to be written specifically for 2 or 4 core processors or will there be a performance increase by using such a processor with my current applications?
By moving to 64 bit OS and say 4 core processor should all my current applications still work?
I know this is basic but my computer is about 8 years old!!!!
Thanks

[#2]

Access to 64 bit drivers spring to mind as a possible problem for older hardware like printers and scanners but this issue is lessening as manufacturers play catch up.

[#3]

There is no reason why any application won't run as the CPU will run in 32bit or 64 bit mode.
If it can't run multi-threaded it will just run on one CPU, with no performance gain.
However, some OS's don't provide proper support for dual mode execution.

[#4]

As Strat says drivers for older peripherals may be an issue.
Also 64-bit add-ins (e.g. Flash) for 64-bit browsers are not yet all available, but 32-bit browsers work (as will almost all 32-bit apps)

[#5]

There are 64 bit browsers which can run 32 bit plugins...

[#6]

Quote from: briston

Do applications have to be written specifically for 2 or 4 core processors or will there be a performance increase by using such a processor with my current applications?

I think the answer is 'yes' and 'yes'.  Applications do have to be be written specifically to take full advantage of multicore processors but as most computers multi-task they can do that better with a multi-core processor so you will see a performance increase.

[#7]

64 bit processors will generally run 32 bit applications ok, but what they won't do is run 16 bit applications. If you have any very old applications, especially those that rely on the Windows Command Shell (otherwise thought of as MSDOS), then they probably won't run.
The Business version of Win 7 comes with the facility to run a Virtual XP machine to get round this problem, as there are still quite a few older business applications out there, but it's not a very convenient solution.
If the new PC is expected to do the next 8 years, then it would seem sensible to go for 64 bit unless there are good reasons not to.