You should be able to order the PC with the exact OS version and hardware that you specify?
I certainly wouldn't settle for a system that only comes with certain, pre-determined options, if it doesn't fit my needs. Call HP and tell them them exactly what you want. Remember the warranty that is included with the new system is only good, if you do not change anything, such as upgrading or installing a new version of the OS.
There is also the subject of drivers, if you order the system with a 32bit OS, does 'all' of the included hardware have 64bit drivers available?
The bottom line is that you don't want to settle for a system that is not 'exactly' what you need. If you do, you will be stuck trying to 'fix it, right out of the box'. I speak from experience! 
When going from a 32bit version to a 64bit version, you must perform a clean install.
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Ronnie Vernon
Microsoft MVP
Windows Desktop Experience