Is it possible that my Dell Inspiron wireless card ( with Blurtooth ) is slowing my wireless broadband by approx 50% from an ethernet connection

With regard to your inspiron 6000 wireless, I believe that the wireless card included is a b/g type. This means that the maximum speed that you could get to a wireless connection is actually 56Mbps which is really around 50% of the normal ethernet connection which is 100Mbps. If you would like to increase the speed of your wireless what you need is to upgrade that to a b/g/n type which goes up to 100Mbps also with increase in distance. Also you have to make sure that the wireless access point or router also has a N-type wireless connectivity. Even if you have a N-type wireless card on your laptop, if the router is only up to G then it will still connect just up to 56Mbps.

Now, with regard to your bluetooth connection interfering with your wireless, You do not have to worry, both these devices connect at a different frequency. So they would not interfere with each other.

Yes, If we consider speed then this is the list. 1. Fiber Optic (Fastest) if connected to RJ 45 ethernet it will be fastest if connected to usb bit slower than rj then comes comm port connection 9 pin then comes bluetooth then comes Infrared connection.