If its for office work I would go for something like a dell desktop. They are built to order so you can spec it to suit your needs and budget. The machine Im currently working on is mainly used for business apps its an AMD Turion Dual core Mobile 2.2Ghz 64bit with 4GB of RAM running windows 7 professional and its rock solid and fast. Its a laptop but it will give you an idea of what will handle the apps you want.
Also if its for an office you need to think about if it will be on a domain. If you need to add it to a windows domain make sure you get it with a "professional" version of XP/Vista/7
