18" laptops eh? A lot of people say that 18" is far too bulky/heavy to put on your lap and will only end up sitting on a desk for its entire life, but considering that you want it for watching movies, then fair enough - bigger is better. On the other hand, you also mention that HDMI is a must... which implies that you're going to plug it into a bigger screen anyway, rendering the 18" screen useless.
So onto the specs. Core i7 may be nice and powerful, but as mentioned, it's overkill for watching movies. You'll find that you can comfortably use a Core i3 and speed will certainly not be lacking. As for the GT330M, it's not nearly bad as phenoM describes, but it's not all that special either. HD5xxx series is the way to go if you plan on doing anything more intensive than watching movies, like gaming, rendering, multimedia editing, etc.
Now for the suggestions. Acer has the Aspire 8942. At the lowest end of the range, it has a Core i3 and a HD5650 and can be found between $800-1100, depending where you look. The configuration with Core i7 and a HD5850 can be found for around $1200-1400. The Core i5 version can come with either graphics card, and the prices fall inbetween the others. They all comes with full economy features (like switching from discreet to integrated graphics to save power) and have a very nice set of touch-control media buttons on one side.
Toshiba Qosmio X505 is similar to the P500 you linked, except that it has a GTS360M and costs are in the region of $1400. Or you can look at the HP Pavillion DV8t, which has Core i7, but only has a GT230M. That can be found for $900-1400.
Otherwise it's somewhat difficult to find 18" laptops. There's a much larger range of 17" laptops with all sorts of configurations, so I recommend thinking about what it is exactly that you want to do with the laptop. Do you really want a "desktop replacement", which'll sit on a desk and never stray far from the wall socket? Or do you want a smaller laptop that's somewhat more portable and can always plug into a TV or monitor to view those movies?
One last point - you want to find a cheaper laptop than what you found. The way to do this is simply to look around at all the different websites that sell them, including eBay and Amazon. You might find a site selling them on discount, or refurbished, or ex-display models, and get over $100 discount from them. Some laptop companies also offer "cash-back" offers if you send them a picture of your receipt. However the best way to find a cheaper laptop is to compromise your standards - look for a 17" or 15" laptop with Core i3 or i5 instead of i7, and you can pretty much chop the price by a third.
