the one for $599 is a much better one. They have different models. The 599 one is:
2.26 Ghz Core 2 Duo P8400 (1066hz fsb)
Vista Home Premium 64bit (all drivers working flawlessly, no conflicts, no problems, FAST!)
320GB HD
4GB DDR2
w/HDMI output
w/eSata2 output
quickplay remote control
etc...
Also, on the HP website: hp (dot) com/bettertogive
you can get a $50 mail in rebate. making the 550=50 and 599=550
=D I bought 4 of these suckaz