Hi Miss Daisy,
Welcome to the forums.
205's can fit, but you will most likely need to either get the guards lipped and/or add camber bolts to give soem negative camber at the rear. 195/50.15's on 15*6.5" rims will generally almost always rub on the rear of an X3 Excel, but the amount of rubbing can be minimised by ensurring the correct offset rim is used.
Also there are some brands/makes of tyres that will rub more than otherss. The 195mm or 205mm width referes to the widht where teh tyre makes contact with the road, and not the maximum width of the tyreitself. Some makes/model of tyres have a very square sidewall, whereas others will have the sidewall that bulges out quite a bit.
Check with your local wheel and tyre shop before burign yoru news rims and/or tyres, and if they are any good they will be able to point you in the right direction to ensure you get the correct wheels and tyres for your Excel with the least amount of hassles.
Cheers
Bretton
www.PerformanceStylingCentre.com.au