Another factor i have is the fact that i do most of my gaming late at night after my parents go to sleep so if im talking really late at night they might get annoyed and with a laptop i can carry it downstairs away from them. (our rooms share a wall)
Unless i was to have my pc downstairs but then i couldnt talk when they were watching tv.
And i wouldnt want to move my equipment to much. So i would either have to move it to another room which is kinda hard to do or i would have to get a lappy.
Otherwise i could try getting a barebones lappy with just a x1400 or so and then get a 20 inch monitor no surround sound and deduct some other things but then i have to deal with really bad laptop performance. Plus the laptop is probably an helpful sort of push for them to get it as it is an investment they could also use
As much as I hate to say it - cause I friggin HATE "Lac-tops" - due to these factors I really think a Laptop is your best bet. I think you need a lifestyle choice, not a performance choice as you need flexibility to get out of the folks way. Once you are out of the "financier" hair then get a desktop, ouch.
Unless of course you can think of a better solutiont hat won't annoy them. Paint their walls.