Yes, dust is bad for a pc's performance. First, dust is an insulator, so it drives heat up. Heat causes extra resistance to the electrical components and they slow down. Next, depending on what's in the dust, it can cause electrical shorting, which can interfere or end the life of those electrical components. And yes, it looks bad.
I regularly use a hand vacuum to clean out the big stuff and a damp dishcloth (only do this when the PC is off and unplugged from the wall) to wipe down as many of the surfaces as I can. Then I use compressed air to blast out what I couldn't reach with the dishcloth, like the CPU heatsink. I have an Antec 900 myself and know how much dust it can inhale. Last note, (no laughing here, at least not too much) I've made some fan filters from my G/F's old pantyhose. The nylon or whatever does a great job at filtering a majority of the crud out of the air while allowing decent airflow.