I'm very suspicious of anything when I'm given the hard sell. The electronics stores spend more effort trying to sell you the $100 cable than they do the TV.
Get it professionally calibrated. The adjustments you can make without going into the service menu (and you do not want to go there) will never allow you to see the best picture you can get.
Apple stores carry an HDMI cable for 20 bucks; if you need to get one quickly locally instead of ordering one for $7, it's a nice alternative to spending 50 bucks on the same thing at Circuit City or Best Buy.