Windows XP is usually the easy win on the later machines here. Where Windows 98 fights the chipset and DOS needs sound-card workarounds, XP is the OS these late Core 2-era boxes were actually built for — driver support exists, and nothing needs coaxing.
The Dell OptiPlex 760 is the clearest example: DOS turned out better than expected and Windows 98 stayed awkward, but Windows XP was the natural fit from the start. The Dell Dimension E520 landed in similar territory — useful for some retro roles, but XP rather than 98 is where it’s most comfortable.
For getting XP itself onto these machines without an optical drive, I’ve used both Easy2Boot and WinSetupFromUSB to build bootable USB installers.