An interesting experiment.
If I remember correctly Microsoft's official minimum requirements for XP Pro were 256MB & a 350MHz Pentium II. But you can have a usable machine below these requirements. I have run XP Pro on a 10 year old laptop for a while (early Intel Celeron, 96MB Ram and a dead slow 4.7GB HDD) and it has been rock solid. It was even perfectly usable for some rather demanding tasks such as mapping in CorelDRAW 12 using multiple layers of a 65MB bitmap and basic editing of audio files up to 1GB in Cool Edit. With XP it doesn't feel noticably slower than Win 98, and it is a lot more reliable.