"What would be the fastest XP Setup with XP Era Hardware?"
Well... first we probably need to define what you mean by "XP Era". Where does your XP Era end? Does it end when the "Vista Era" begins? Does it end when Microsoft stopped supporting it (Well into the Windows 8 Era)?
Windows XP Release - October 25, 2001
The fastest CPUs were the Intel Pentium 4 "Willamette" 1-core @2.0 GHz and the AMD Athlon XP 1800+ "Palomino" 1-core @1.53GHz.
The fastest graphics cards were the NVIDIA GeForce3 Ti500 and the ATI Radeon 8500.
Windows Vista Release - January 30, 2007
The fastest CPUs were the Intel Core 2 Extreme X6800 "Conroe" 2-core @2.93 GHz and the AMD Athlon X2 64 6000+ "Brisbane" 2-core @3.1GHz .
The fastest graphics cards were the NVIDIA GeForce 8800 GTX and the ATI Radeon X1950 XTX.
End Windows XP Mainstream Support - April 14, 2009
The fastest CPUs were the Intel Core i7 Extreme 965 "Bloomfield" 4(8) cores(threads) @3.2GHz and the AMD Phenom II X4 940 Black Edition 4-core @3.0GHz.
The fastest graphics cards were the NVIDIA GeForce GTX 280 and the ATI Radeon HD 4870 X2.
Windows 7 Released - October 22, 2009
Windows 8 Released - October 26, 2012
Windows 8.1 Released - October 17, 2013
End Windows XP Extended Support - April 8, 2014
The fastest CPUs were the Intel Core i7-4771 "Haswell" 4(8) cores(threads) @3.5GHz and the AMD FX-9590 8-core @4.7GHz.
The fastest graphics cards were the NVIDIA GeForce GTX TITAN Z and the AMD Radeon R9 290X.
There you go.