I'll echo the recommendations for an iMac 27" and MacBook or MacBook Pro. That new 27" iMac is SWEEEEEET! Particularly if you get the four-core model! That baby is pretty much the same power as my 4-core Mac Pro and that 27" LED-backlit screen is fantastic! Lots of room to spread out with Aperture, Lightroom and/or Photoshop!
For the laptop, I'm biased to the MacBook Pro. USB is fine for printers and keyboards and mice and such. But for external hard drives I much prefer Firewire! I also love Firewire for transferring my photos. I use a SanDisk Firewire 800 UDMA CF reader and it is very fast when used with a UDMA CF card. No USB card reader can touch it for speed. And when I'm sometimes transferring 1,000 5D Mark II photos (25MB each) in a sitting, yes, the speed matters!
The good news is, once you go Mac, you won't go back!
