There is any advantage in Win XP 64 bit?

I strongly recommend against Windows XP on 64 bits. It has NO advantage over Win XP 32 bit unless you have more than 3.2GB of RAM.If you have 4GB of RAM, Win XP 64 bits will give you back those 0.8GB of RAM that Win XP 32 bit cannot access, but at the same time you will instantly lose that amount because (I suppose you already know that) programs on 64 bits requires much more memory than programs on 32 bits. This is because

now the program requires 8 bytes (64 bits) to store its internal data, instead of 4 bytes (32 bits), which is double amount of RAM!

So, you gain nothing unless you have way over 4GB or RAM (for example 6 or 8GB). Even worst, Win XP 64 bits has massive compatibility problems!! If you MUST use an 64 bit OS, then upgrade directly to Windows 7.

Before switching to 64 bits, take a paper and a pen and do the math. See what you actually gain in order to compensate for the horrible compatibility problems a 64 bit OS has. Better install the OS on a spare HDD and play with it. If it is stable enough, then you can switch to it definitively.