Like others have said, decide on your budget. The room design is one of the most important things when building a dedicated HT. If you want good sound, start with the room. When running the wiring, use PVC or some other type of conduit so that wires can always be changed or added later on.
Figure out what type of display you'd most like to have. Front projection will give you the largest screen. FP's are not cheap. Sure you can find one for $1,000 dollars but a good one will run $4,000 to $10,000.
Screens can run: $400 - $2,000
Audio components will cost around: $2,000 - $5,000 (it can be done cheaper but this is for some GOOD equipment)
Speakers: Figure $4,000 - $9,000
Cables: $500 - $2,000
Seats: $300 - $1,000 each
Misc: $50 - ????
Figure out what type of display you'd most like to have. Front projection will give you the largest screen. FP's are not cheap. Sure you can find one for $1,000 dollars but a good one will run $4,000 to $10,000.
Screens can run: $400 - $2,000
Audio components will cost around: $2,000 - $5,000 (it can be done cheaper but this is for some GOOD equipment)
Speakers: Figure $4,000 - $9,000
Cables: $500 - $2,000
Seats: $300 - $1,000 each
Misc: $50 - ????