So my genius brother has taken it upon himself to BUILD his own computer despite knowing nothing about building computers. The task has fallen on me to finish setting up what he couldnt.

He felt the need to completely revamp our internet also, which wasnt broken but i suppose NEEDED to be fixed anyways. He has gone out and bought both a new router and a card for his desktop. MY desktop (joel-pc) is now hardlined into the router, and his desktop (sean-pc) has the card in it.

-the router is an Asus RT-N53
There are multiple computers already on the network, working fine:

joel-pc: running windows 7, hardlined into the router, working fine
joel-netbook: running windows xp, uses generic w.e hp decided to use for a card, working fine
sean-pc: the idiot-build computer, running windows 7, has a d-link DWA-552 Xtreme N card, will not connect for the life of me; it wont even discover either band of the router, yet it occasionally will pick up our neighbors connections which are farther away than the router
sean-netbook: running windows xp, uses w.e manufacturer decided to use for a card, working fine
mandy-pc: running windows 7, was a store-bought all-in-one desktop pc and has some integrated wifi card from hp, working fine
danny-pc: running windows xp, uses a d-link card (not sure on exact model), working fine
There are also 2 PS3's which connect wirelessly (albeit very slowly) without problem.

i tried changing our channel width on the router from AUTO to 7 (highest it offered) and it didnt work for the desktop (sean-pc). Im not particularly savvy when it comes to networking issues, so i have no other idea what to do to change it. I manually downloaded the latest drivers from dlink, manually installed those for the card, and still, nothing.

mandy-pc was able to seamlessly connect automatically as well as discover the network with no problem, so the only other distinguishing factor apart from the difference in hardware is the fact that every computer with the exception of sean-pc and sean-netbook, is on the same floor as the router.
My brother assures me however that he has bought "top of the line" hardware for both the router and his card, and that the fact that its on a different floor should not matter.

Additionally, if this might help the diagnosis, I originally followed all instructions that came with the Asus router, figured to have set it up fine. Im having no real problems with all other computers, other than the fact that our internet seems to be pretty slow. I only notice the SLOWNESS when my brother and i try to party-up on call of duty on our ps3's, when our connection usually seems to go to crap.
The router is dual-band, so i set up a 2.4g network which all the computers in the house use, and a 5g network which i left the ps3's to connect to (the Asus setup utility claimed this was optimal as the 5g was supposed to be "specialized for gaming").
Where im going with this is that PERHAPS our slow connection/my using of the 2 bands might have been improperly configured (despite every other device getting at least SOME connectivity) such that my brothers d-link DWA-552 is unable to pick up the router?

any-all help would be greatly appreciated. Have you tried the card in another computer?
Have you tried to "Manually connect to a hidden WiFi network" in Windows 7 and manually enter in the router's SSID?

What are the specs of your brother's computer? (I'm asking this because I once had an issue with an underpowered computer causing a Realtek RTL8185 PCI WiFi card to frequently drop connections due to signal issues and causing the computer to crash whenever more than 2 USB devices that drew their power from the USB port were plugged in)
Your brother's PC could be in a dead spot just out of range of the wireless coverage-try to move it AROUND the room and see if the network is picked up.hi yes the card works in other computers and i dont think its a deadspot because his netbook and old computer both had internet in the same spot

Computer specs:

os: windows 7 home premium 64-bit (6.1, Build 7601)
Bios: Bios Date : 02/05/10 19:13:52 Ver: 08.00.10
Processor: Intel(R) Core(TM) i5-2500k CPU 3.30 GHz (4CPUs), ~3.3 GHz
Memory: 8192 MB RAM
DirectX version: DirectX 11
GPU: Nvidia GTX 570 (4064 MB)
PSu: coolermaster silent pro 850 watt
motherboard : Asus p8z68-vpro
monitors: 2 samsumg led
Network adapter: D-link DWA-552 XtremeN Desktop Adapter
SSD: OCZ vertex 3 60 GB
HDD: western digital velociraptor 300 GB
Samsung 12x blueray drive
Speakers: Realtek high def audioWe can rule out your PSU as it is more than adequate for your system power needs. We have also ruled out the possibility of the card being unable to detect your 2.4 or 5 GHz band-it works in other computers.
Have you tried the card in both of the 2 PCI slots that your motherboard has? Have you installed the latest D-Link drivers?(Windows does have default drivers which may or may not work properly with your card)
Have you tried what I suggested above, that is http://www.7tutorials.com/how-connect-hidden-wireless-networks and enter in your router's SSID?Are you setting up the router yourself or are you using all those utilities that come with the router?
What kind of wireless encryption are you using? And what settings have you changed in the router?
Did you shut down any torrent software?

Personally, I'd have restored the default settings by now and simply started over from scratch.
Especially if the router can't be detected wirelessly and if network connections are slow.