If I installed Windows 7 64-bit on an Intel core duo machine, will this cause any problems?
It's a 3 year-old computer, so I'm not sure if it will work.
Is it a Core 2 Duo or Core Duo?
If it's a Core 2 Duo it's fine.
If it's a Core Duo, it won't work, because the Core Duo is only a 32-bit processor.
You should also check that 64 bit drivers are available for your internal components (graphics, network card, etc). Check out the microsoft site for tested components and I think they also have a downloadable tool that will check your machine for Win7 suitability.