Just get the Mini. You'll be glad you spent an extra $179 (not much in the grand scheme of things) to get what you really want. I'm enjoying my Power Mac a great deal and don't regret the purchase in any way.
Besides, with Intel Macs now able to reliably boot and run XP thanks to Boot Camp, you can switch if you're ever in the mood to do some fragging or play Solitaire. I still have a Windows box I keep around for online gaming, but once the Rev. B Intel Power Macs are out next year, I'll be kissing that PC goodbye.
I say take advantage of the opportunity you've been given to do what you've always wanted to do. Life is too short.