Here is a list of acceptable 16" sizes for Scion XD, according to Discount Tire Co.:
I think the 205/60/16 would look great, it would give the XD more of a mini-SUV appearance. The allowed rim width for that tire size is 5.5"- 7.5", so you are good to go with the factory 16x6"steel wheel.
*EDIT: Some of the sizes listed will require a 16x6.5" or 16x7" rim for proper fitment/performance.
I have 205/55/16 tires on mine It's only about a 1mph difference at 70, so you'll hardly be able to tell the speed difference.
I got Falken Ziex ZE-912 tires and they've worked well for me so far. MUCH quieter than the OEMs and have the best heavy rain grip I've ever experienced. $108 per tire as well. They'll probably only last you 30k miles, though, so read up on the conditions of your tire mileage warranty to get some money back.
205/55/16 will give you the best variety and will be the cheapest. 215/60/16 is also a good step up and will help with the stupidly low gear ratios of the car, but I wouldn't go for 225 widths since they'll look way too wide on a 6" rim.