Vista 64 bit wireless nightmare

Hey everyone. I'm having a lot of trouble with my wireless connection on my 64 bit vista machine.
It's been happening for a while and it is driving me absolutely crazy. In my old house I had a wired internet connection but have now moved into a house where I have to have wireless. I need the internet to for my work so at the moment this problem is having a massive impact on that. I can't tell you how much I would appreciate someones help with this.

Firstly here's the spec:

Asus p6t delux
i7
6gb ram
d-link dwl-g510 wireless card
64 - bit vista sp2

the router in using is D-Link DIR-615

Now here is the problem:

The computer finds the wireless network and connects with it perfect. The connection never drops and is always at full strength. However the internet is either incredibly slow or just doesn't work. It depends what you try to do.
Some sites (google, engadget...) loads at average speed but most don't load at all, time out or load after literally 15 minutes.

Downloads from web browsers dont work

spotify and itunes sometimes think they have a connection but all downloads fail.

When I connect to the router with an ethernet cable everything works great and downloads speeds are amazing (16mbps!!).

What I've tried

Tried multiple drivers for the wireless card (now running the most up to date one)
Tried a different wireless card (had exactly the same problem)
Tried changing pretty much all the settings on the router, changed all the security settings.
tried disabling all the security software on my computer.

If you have any ideas or suggestion at all then please please let me know. I tried posting something similar in the wireless forum but got no reply. Thanks in advanced.

Arges86 thanks for your reply. I don't think that the issue is port forwarding because any other computer or device that connects to the router over wifi has no problem at all. Anyone else got any ideas?