Erm, no, again I don't think this is correct. Those "reserved" memory spots you see in device manager are slices of system RAM that are set aside for use by those hardware systems.
"PCI Bridge Device, IDE Controller, USB Devices, On-board Audio"
Aside from MAYBE your audio card, none of those have additional RAM ICs attached to them to ADD memory top the system, they just subtract available memory from system RAM.
If you look at the MS paper
here, please note "Windows XP Professional x64 Edition supports up to 128 GB of physical RAM and 16 TB of virtual memory. By comparison, 32-bit Windows is capable of supporting up to 4 GB of system memory, with up to 2 GB of dedicated memory per process."
System memory does NOT include memory on other devices (such as video cards, RAID controllers, SCSI cards, audio cards, etc.).
My comments on video memory, however, fall apart when you use onboard video with Shared Memory Architecture (SMA) which is rare these days. The issue ius partially due to the fact that the system splits memory into 2GB app and 2GB kernel. PCI devices ALWAYS steal a little memory. Another card will steal memory no matter what it is or if it has onboard memory or not.