WinXP64 will install all your 32 Bits softwares in c:\Program files (x86)\, and they will work without problem (XP64 emulates the 32 bits)
All the 64 Bits softs will be installed in c:\Program Files\
Except for a few sharewares, i personnally had no pb
Choose a good motherboard that will allow you to easy and safely overclock your quad (like the Gigabyte GA-P35-DS3R (rev. 2.1) ), and good Ram with low latency, like G-Skill for example.
An Antec Sonata III is also a good choice, it contains a 500W Enermax alim
I let you choose the 3,5'' drive
