Wireless setup dilemma

Hello guys, my name is Alex.
The trouble with my Home Wireless Setup is Speed, and I am unable to find the Cause of the flaw in the Setup. Here are the components:

- Wireless Router D-Link DIR-635 Wireless N
- D-Link DWL-G520 PCI Wireless Adapter:
- my PC using Win7 (old Intel E6750 with the above PCI wireless adaptor)
- my girlfriend`s laptop:Toshiba Satellite L300-11C
- a PS3 slim 160 GB

the problem:
my total wireless download speed is capped at 2.1 MB/s. This means:
- If I have 1 download, it will run at full speed with 2->2.1 MB/s. At this point the IE(I use Chrome) web pages will Not load, or they will take Ages to load.
- if I have 2 or more downloads running, one will be ~1 MB/s and the other around 1 MB/s. So the same combined speed. Same situation with the IE web pages, they will not load.
-------------> now this tells me that there is a maximum of 2 MB/s that my router can send me. Am I wrong ?

I have tried:
- using a WIRED connexion from the router to My PC, resulting in a speed of Max 4MB/s of download. Same thing for the Web pages, they will not load, if this speed cap is reached. (this solution is not good, the Router is in 1 room and the PC in another. this cannot be changed. the Wired test is only a temporary solution, for debugging this issue.)
- I have asked the guy from the ISP, and he told me the issue is somewhere in my setup, and I should have 5-6 MB/s.

what I Think:
- I realize that the ISP guy may not tell me the whole truth, and that I could have a maximum of 3-4 MB/s (which would be Fine, it I could get that Wireless)
- I have not read all the forum, only posts from the first 4-5 pages, but I have found no solution yet.
- I think I`m missing some setup tweak from the Router? It is set at 20 MHZ - Auto (something).

My Goal is to have around 4 MB/s while using the Wireless connexion.

Thank you all for your time.