I went through the same dilemma. I came from the following PC Setup:
AMD Athlon 64 4200 X2 (Dual Core)
2gb RAM
ATI 1900XTX Graphics Card
SATA Drives in RAID 0
To the following:
Mac Pro 2.66
2gb RAM
7300 GT (ATI 1900XT On Order)
My happiness meter? 10+.
I love this machine. OSX is very nice, it runs the games I play (well, ok, World of Warcraft) and ultimately, I like what Apple has done. Plus, if I ever wanted to, I can go back to Windows anytime I want either through Parallels or Bootcamp.
Oh, and we'll need a better description of what you want it for other than "powerhouse" to be able to give you any good advice. i mean, if you're just messaging and checking email, then the mac mini would be a powerhouse for you..
Do you NEED a powerhouse? I got my Mac Pro for Aperture and Photoshop work, and even I'm not sure I need all this power. Looking back, I should have waited and bought a 24'' iMac. Could have gotten it with 2 GB of RAM, 500 GB HD, nVidia 7600 GT and saved myself $700. Oh well. That's what I get for being impulsive...