Oldows support for something like that might be dicey. I wonder why one would just not run 32bit(better compat) windows server 2003. A bit of setup for desktop use for sure. That said, if you are going down this road, you are already rolling your sleeves up.
list "(USB) Floppy Diskette Drive" as one of the hardware requirements.
As for storage, the Z820, at least, includes an XP-compatible LSI 6Gbps SAS controller onboard in addition to SATA, so performance with SAS and/or SATA drives shouldn't be an issue.
High core count, dual-socket Z820s are readily available on the used market for <$1,000, and seem like they'd make excellent high-end XP boxes (and even better Windows 10/Linux boxes; I'm running both under ESXi with GPU passthrough on mine; licensing issues aside, current [10.14 and 10.15] OS X versions also work, including GPU passthrough, at least in the case of the NVIDIA Quadro K2000 and Apple-supplied drivers).
More generally, XP-compatible 6Gbps SAS PCIe RAID cards and HBAs are readily available on eBay for peanuts, so that'd probably be the way to go if you want the best disk performance under XP with (relatively) modern hardware.
USB floppy drives work as designed to install SATA drivers during XP Setup.
You push the F6 key when prompted just like NT Setup where you would add SCSI drivers if needed from a manufacturer's OEM floppy.
The OEM (F6) floppy for your specific SATA hardware will contain a fileset with their basic drivers and a TXTSETUP.OEM text file referencing that particular Device ID as well. These floppy contents are often included in a small folder in the manufacturer's full unzipped SATA driver setup files.
There are workarounds if you don't have a floppy, see my other message.
Any proper BIOS serves USB devices to DOS no differently than legacy hardware, even if it is optional to be enabled when needed. You do have to plug in USB drives, kybd, mouse before turning on the PC but then DOS or W9x can boot from or access them best usually without USB drivers installed in the OS. USB-connected non-80's-legacy devices like webcams would be recognized by W9x only if the USB hub they were on had been properly installed beforehand to the somewhat uncommon USB-enabled W9x, but still would not work unless you had the device-specific W9x USB drivers for windows installation of the exact cam. Cams like this were the kind of thing that actually used windows drivers and would work after windows was booted without having to plug them in beforehand.
I don't have one. What kind of device interface do USB floppy drives use, and does Windows XP have drivers? I highly doubt it (edit: others have pointed out that they use USB-FDD mode and is supported by Windows XP, so yes.).
The reason I don't have one is that although I have floppy drives, but I cannot find a single working blank floppy anymore... A floppy emulator that uses an SD card and the identical floppy interface should have better compatibility (but without the convenience of Plug & Play like USB), and you don't have to find a source of working floppy.
IIRC even a USB drive in HDD mode with the appropriate drivers (they were different based on chipset) will work. It's been a long time since I've done it, but I remember installing the AHCI driver from a USB drive or even a partition on the internal hard drive.
Later on I found you could integrate the driver into the WinXP installation CD itself.
Might work on motherboards with a bIOS implementation, there’s often a “legacy” mode that emulates a FD interface making a USB floppy bootable and accessible from Windows Setup. Likely not the case for any UEFI motherboard.
All recent[1] non-UEFI (BIOS only) and UEFI/BIOS hybrid systems I've used provide BIOS emulation for both floppy drives (using USB floppy drives) and hard drives (using USB mass storage devices; e.g., I have MS-DOS 6.22 installed on an MBR-partitioned USB flash drive that I regularly use to flash firmware updates, etc.).
Pure UEFI ("class 3") firmware without BIOS support won't boot (32-bit?) XP at all; as for whether native UEFI drivers are typically provided for USB (or any other) floppy drives, I have no idea.
[1] Qualification because I recall having trouble booting from USB optical and flash drives on at least some USB-capable systems that predate XP, but haven't had a problem with USB boot on any system in at least a decade.
Oldows support for something like that might be dicey. I wonder why one would just not run 32bit(better compat) windows server 2003. A bit of setup for desktop use for sure. That said, if you are going down this road, you are already rolling your sleeves up.