I know for a FACT that the limit for 32 bit Windows XP is 4 Gigs of ram, it recognizes a little bit less. The 64bit version of XP (And Vista) has a limit of 128GB of ram, but that's irrelevant, since you are never going to find a end user PC motherboard that supports that amount. 64bit XP is a bit more fussy than 64bit Vista in general, and there is less support on 64bit XP than there is 64bit Vista.

Now if this is a laptop, that particular model's physical RAM limit may have already been met.

And no, you can NOT run an "upgrade install" of XP 64 bit over XP 32 bit, you must do a CLEAN install (format and install), according to various MICROSOFT sources:

Q. Can I upgrade from 32-bit Windows XP Professional to Windows XP Professional x64 Edition?
A. No, you can't upgrade. You can use the Technology Advancement Program until the end of July, 2005, to exchange your copy of Windows XP Professional for the x64 Edition of Windows XP Professional, but doing so replaces your existing license with the new license. The actual installation of Windows XP Professional x64 Edition is a fresh install, not an upgrade. All your programs will have to be reinstalled, and if you have data on the same drive as your operating system, it will be wiped out. Make a thorough backup of your important files before doing the installation.
Source: http://www.microsoft.com/windowsxp/u...el_x64faq.mspx

The Ferrari 4000 ships with a 100 gigabyte hard drive, partitioned and formatted into two equal logical drives of around 45 gigabytes each and a small hidden "factory restore" partition. The factory installed applications were all on Drive C and I installed additional applications on Drive C as well. Drive D was still empty. Two options were available to me to install Windows XP Pro x64:

1. I could format Drive C and clean install x64.

2. I could set up a dual boot and have both 32 bit and 64 bit Windows XP on the same machine on different partitions.

A dual boot option seemed like the safest option, given the lack of vendor supplied 64 bit drivers at that time. I also was not certain if all my 32-bit applications would run on 64 bit windows. I knew that with a dual boot system, I'd be able to use the pre-installed 32-bit Windows XP Pro operating system if I failed to find needed drivers or had application issues.
Source: http://www.microsoft.com/windowsxp/u...n_05nov07.mspx

Installing a 64-bit version of Windows Vista on a computer that is running a 32-bit version of Windows XP or of Windows 2000
Most Windows XP and Windows 2000 users have the 32-bit version of these operating systems. For example, Windows 2000, Windows XP Home, Windows Tablet Edition and Windows Media Center Edition only come in 32-bit versions. For these operating systems, there is no upgrade installation path available when you upgrade from a 32-bit operating system to a 64-bit operating system. However, you can perform a custom installation...
Source: http://support.microsoft.com/kb/932795

Check your facts before you insult someone who does this for a living.