Broadcom Netxtreme 57xx problem

I have a Optiplex GX280 running WindowsXp, service pack 3.  I recently installed a 3rd party pci ethernet gigabit adapter because I am unable to connect via the onboard broadcom controller at more than 50mbps (even though the onboard ethernet is supposed to be a gigabit controller).  The duplex speed said the onboard controller wouldn't go above 100mbps, so I switched it to autonegotiate and still no faster than 50mbps. 

My new card will allow me to have full access of the different speeds up to 1gbps and is currently set to autonegotiate.  However, I still have the same problem of not being able to get faster than 50mbps.  I also just bought a new Arris dual band wifi router which the optiplex is hard wired to.  When I hardwire my laptop into my router (which only has a 10/100 adapter), I get close to 90mbps.  So there is definately an issue with the desktop.

I have been reading a lot about the different cat cables that I should and shouldn't use to get the most out of my adapter.  Some site say do not use cat6 (only use cat5e), and some sites say it doesn't really matter.  I currently have cat6 plugged in (I'm waiting on a cat5e that I ordered).  Is this the reason I can't get the speed that I should be getting out of the gigabit ethernet card?  I know I'm working with older technology but even the onboard ethernet (which is a gigabit controller) is not putting out the speed it should be getting.  Any help would be grateful.  Thanks.