“How rational will it be to recompile a project for a 64-bit system?” – that’s a good question, so take your time to answer it and think it over. On the one hand, you might lag behind your rivals failing to offer 64-bit solutions on the market. On the other hand, you might waste your time developing a 64-bit application that will have no competitive advantages.
Here are some factors that will help you make a choice.
Application life-cycle
You should not create a 64-bit version of an application with a short life-cycle. The WoW64 subsystem allows obsolete 32-bit applications to work rather well on 64-bit Windows systems. It is unreasonable to make a program 64-bit if you stop maintaining it in 2 years. The practice shows that the move to 64-bit Windows versions will be very slow and smooth. Perhaps most of your users will use only the 32-bit version of your program solution in the nearest future.
If you plan a prolonged development and maintenance of your program product, you should start working on its 64-bit version. Of course you should take your time but keep in mind that the later you have a full 64-bit version, the more problems you are to encounter while maintaining such an application installed on 64-bit Windows versions.
Application performance requirements
After being recompiled for a 64-bit system a program can use huge amounts of memory and its speed will increase in 5-15%. 5-10% of speed gain is achieved due to architectural features of the 64-bit processor, for example, a larger number of registers. And another 1-5% performance gain is determined by the absence of the WoW64 layer that translates calls between 32-bit applications and the 64-bit operating system.
For example, Adobe company says that their 64-bit “Photoshop CS4″ was 12% faster than its 32-bit version”.
Applications involving large memory amounts can expect a great performance gain. These are graphical editors, CAD-systems, GSI CAD, databases and packages for modeling various processes. The capability to store all the data in memory and therefore avoid additionally loading them from the hard disk may increase the speed of such applications not by several per cent but by several times.
Using third-party libraries in a project
Before planning the work on developing the 64-bit version of your product, find out if there are 64-bit versions of libraries and components it employs. You should also find out the pricing policy regarding the 64-bit versions of the libraries. All this you may learn on the site of library developers. If there is no support for the libraries, search for alternative means supporting 64-bit systems beforehand.
Dependence of third-party developers on your libraries
If you are developing libraries, components or other items intended for third-party developers to create software with, you must be quick in creating the 64-bit version of your product. Otherwise, your customers who are interested in 64-bit versions will have to search for other solutions. For example, some soft- and hardware security developers appeared to be very late in creating 64-bit programs and it made some of their clients choose other tools to protect their software products.
There is one more benefit of releasing a 64-bit version of a library: you may sell it as a separate product. Thus, your customers who wish to create both 32-bit and 64-bit applications will have to buy 2 different licenses. For example, Spatial Corporation company sticks to such a policy when selling their library Spatial ACIS.
16-bit applications
If your solutions still have 16-bit modules, you must get rid of them. 64-bit Windows versions do not support 16-bit applications.
Assembler code
Do not forget that presence of large assembler code fragments make it much more expensive to create the 64-bit version of an application.
Toolkit
If you have decided to create the 64-bit version of your product relying on the factors mentioned above and are ready to spend time on it, the success is not guaranteed yet. You should have all the necessary tools for that. While making a decision, please keep in mind the last very important factor we have not mentioned here: the price of modifying your program code to compile it in the 64-bit mode.
How Not To Code 