Since it hasn't been said yet... High Definition Display on every machine.
Oh yea, almost forgot the most important thing: It would have an autographed picture of Steve Jobs with it, and a confirmation that one will be sent to benixau because he wants one too... he just doesn't want to admit it.
Well, just like my modular laptop, I think we need modular desktops. the PCI bus should be entirely modular. You order the number of PCI slots you want, and if you want more, buy a new module. You should be able to buy as many pairs of processors you want, and even a single processor for the low end. Start out with a dual 2ghz, and if you need more power, buy another 2 2ghz processors etc. The case would also be modular, so if you got the minimum configuration, it would basically be a cube, but with the ability to expand to a 8 processor super station.
17" imac, AO5 dvd-r drive, 52x24x52 cd-rw (all in the base mind you), AGP expansion, radeon 9700, 240 gig HD, t3 connection, 5.1 sound, some case mod somehow, 4 3GHz cpus by IBM or AMD, and some hack to make it double as a true PC for games, horrid as that may sound. All for $500