i did it once, changed from a celeron 2.4 Ghz to a Pentium 4 2.6 Ghz, had no problems. Same OS, XP Pro w/SP2.
only concern is that it might be different now with newer components.
i recommend just backup your files and give it a go, if it doesn't let you just fresh install.
CPU: Intel Q6600 Quad @ 2.4 Ghz per core
Motherboard: ASUS P5B-Deluxe wifi
Hardrives: IDE MAXTOR 80 Gb (Windows)
2x SATA MAXTOR 320 Gbs in raid
Video Card: Nvidia 8800GTS (revised 512mb)
RAM: 6 Gb 667 kingston dual channel
Monitor: ASUS VW192T
Mouse: Microsoft Habu
Power Supply: Topower 1000W
OS: Windows XP x64 edition (to support all ma ram :D)