Pentium 233-megahertz (MHz) processor or faster (300 MHz is recommended)
At least 64 megabytes (MB) of RAM (128 MB is recommended)
At least 1.5 gigabytes (GB) of available space on the hard disk
CD-ROM or DVD-ROM drive
Keyboard and a Microsoft Mouse or some other compatible pointing device
Video adapter and monitor with Super VGA (800 x 600)or higher resolution
Speakers or headphones
These are the MINIMUM for XP home I presume XP Pro would be similar however you asked what the ideal is and thats differcult if not impossible to answer, ideal for what? It depends what you're doing. As a guideline I would suggest at least a 600mhz processor for speed and at least 256mb RAM and at least a 20 gig hard drive although it will run providing you meet the above minimum requirements. The faster the better and the more memory the better as XP really eats up the memory.