When you stream over the wireless, you are running into a bandwidth problem in the higher resolution files. They require more data to be passed per second than the smaller resolution. That is why you see better performance when you pull them down locally onto the laptop or are running DVD quality files. If you were to plug into the network with a wired connection you would be fine.
The second scenario where you can't seem to get the 1080P files to play even when they are on your local hard drive is most likely caused by the video card. With this being a laptop there isn't much that you can do about that, but there are two things I would check. First, make sure you have the latest and greatest driver for that particular card. Second, try allocating more memory to the card through your BIOS settings. I would put 1 gb towards it myself. They claim that card should be "sufficient" to play HD quality stuff, but I have run in to a similar problem myself and unfortunately sometimes they just don't live up to the hype.
Sorry if that wasn't the result you were hoping for, but aside from double checking a couple other things in person I can't see any other reasons why you would be having that problem.
Cool thanks, I will try the memory thing.