I think in the first place it is a bad idea to put your TV on top of the Dell monitor. Like that, the TV is going to be too high for any comfortable viewing angle. One hour of movie your neck wanna break already. Besides, putting the two LCD together will cause unwanted distraction from each other.
Computer monitor is supposed to be view from very near distance, whereas LCD TV is supposed to be view from a few meters away. Put them together, you want to sit near or sit far?
If you ask me what is the best placement, I would say move the Dell monitor and both your computer tables away. Leave only the TV in the middle, front speakers at left and right of TV, and the amplifier at the bottom of the TV (either on the floor or a very low TV rack/shelf). Don't use your computer tables near the TV coz the nice look of the TV + Home Theater set get spoiled by the table color. Sub-woofer can put at the corner of the wall (the current spot of your front left speaker now).
Lastly, put your Dell monitor and computer and computer tables in the opposite corner of the room/hall. If you need to connect your computer to the TV, use a really long HDMI cable.
For the wall mount, you want to get what type? Fixed angle? Adjustable angle? Full swivel arm control? They are all in different price range. What is your budget? Are you going to DIY or get qualified ppl to install for you?
To get an idea for pricing, check out the user "yahoong" in Lelong.com.my, he has several types complete with pricing