I run Windows XP Pro daily in Parallels Desktop on a 15.4″ 2.16GHz Core 2 Duo MacBook Pro with 3GB of RAM and a 120GB hard drive. It runs just fine for how I use it (browsing VPN shares, editing Word document user’s guides, building things in Visual Studio. It runs great. Nothing I’ve tried feels slow.
I recently bought a backup notebook — a 15.4″ Dell Vostro 1000. 1.72GHz Athlon X2 with a 120GB hard drive running XP Home. I’ll tell you, XP feels faster in Parallels Desktop than it does on the Vostro. Nice!