@Morgano and Xenophonz
Thanks for replying. I only saw the previous posting after I posted this thread. It is good to know that all the "important" software will continue to work. I guess I will try the large address aware tweak for the most memory hungry 32 bit applications (since XP pro x64 is already large address aware).

@flibbits
I built the PC from individually purchased components (see below) because I do not trust the clone PC manufacturers to always use "the best" or even "good" components. In the past, I have purchased several PCs that I found to have some really poor and/or inappropriately underspecced components inside.
So far I have spent £860 (about $1720) on all the PC bits and the 24" monitor. I already had 2 x 2GB of memory and a spare laser mouse. The Win XP Pro x64 license was £85 ($170).
So, I estimate that the total cost would be £1039 ($2078).
I am sure that I could find a cheaper PC with similar specs, but I really wanted a fairly small case and good performance without using too much power.

X-Qpack II Micro ATX Gaming Case with 500W ATX2.0 PSU
Intel Core 2 Duo E6850 "LGA775 Conroe" 3.00GHz (1333FSB)
2 x Samsung SpinPoint T 500GB SATA-II 16MB Cache (HD501LJ)
Gigabyte GA-G33M-DS2R Micro ATX (Socket 775) PCI-Express DDR2 Motherboard
Samsung SH-S203NBEBN 20x DVD±RW SATA Dual Layer Lightscribe ReWriter (Black)
OcUK 4GB (2x2GB) PC2-6400C5 800MHz DDR2 Dual Channel Kit
OcUK Value (Digimate) L2442W 24" Widescreen LCD Monitor - Glossy Black
Cherry Black CyMotion Expert (USB/PS2) Keyboard
HIS ATI Radeon HD 3850 Pro Turbo X 512MB GDDR3 TV-Out/Dual DVI/HDMI (PCI-e)
Arctic Cooling MX-2 Thermal Compound
Zalman CNPS8000 CPU Cooler (Socket 754/939/940/AM2/LGA775)
OCZ gold 4GB (2x2GB) dual channel DDR2 memory kit (pc6400, cl=5)
Microsoft laser mouse

This PC seems to have some overclocking potential since the CPU is operating at about 35 degrees C. The power draw from the socket (excluding the monitor) is 145 Watts under moderate processing load (i.e. one core at 100%).