I don't know how old your PC is but you should be able to easily connect your PC to your new iMac via the ethernet port and transfer all of your files or you can do it with a CD if you prefer. There's also a piece of software called Move2Mac
http://www.detto.com/mac-file-transfer.html?gclid=CIDK-qTot44CFRNkYwodHXKIww.
I would really recommend using iWork rather than the current version of Office. 2 reasons, iWork is optimized for the new Intel Macs so it will run a lot faster. The iWork suite is feature rich and my PC friends really like when I send them spreadsheets created by Numbers plus the Pages handles XML files.
The iWork suite is fully compatible with Office 2003 and 2007 with an exception of a small feature which is best suited for corporate file exchange.
The cost for iWork is only $79.00 and comes as full working trial version on the iMac.
If you want MS Office it's best to wait until Office for Mac 2008 is released in January. I would recommend ordering your iMac with the wireless Mighty Mouse.