Mac Pro tackles demanding computing tasks with lots more style than any Windows-based tower, go for the mac pro if you still want to experience a desktop style pc.
iMac 2.8GHz i7
CPU i7 Upgrade: $200
Memory (4 x 2GB from Apple): $200
Hard Drive (1TB from apple, not user replaceable): included
Graphics Card: included
External 2TB backup system: $280 (OWC Mercury Elite-AL Pro)
Anti-Glare film (ClearCal): $40
Notes: No graphics future card upgrade. Backup is external. No way to add internal cards for eSATA or anything else. Screen somewhat compromised by anti-glare film.
Mac Pro 2.66GHz Quad-core Xeon
CPU i7 Upgrade: not needed
Memory (4 x 2GB from Apple): $250
Hard Drive (640 from apple, 1TB from 3rd party): $80
Graphics card: $200
Internal 2TB backup drive (third party): $175
30" LCD Monitor: $1200
Notes: Large CPU box. Larger anti-glare monitor. RAM and card expandability. No camera or Mic.