I have a DELL Vostro 1500 running Windows Vista Home edition on 32-bit. How can I increase to 64-bit?
Microsoft answers a few of the more fundamental questions on this support article - http://www.microsoft.com/windows/windows-7/compare/32-bit-64-bit-faq.aspx
Look up the answer to - Can my computer run a 64-bit version of Windows?
If your processor is capable of 64 Bits, then all you need to do is reinstall the 64 Bit version of Windows. The Windows support article looks into that. Here's an eHow post detailing the process in brief.
When considering an upgrade also check whether the apps you regularly use have 64-Bit versions or not.