You're doing great! Here's the checklist from this site and you've got all the basics covered:
http://www.redearslider.com/startup.html
Yes, you'll need rings or stars. The bio media basket on the XP3 takes two packages. You can use either rings or stars. I prefer rings because the stars float away when I'm cleaning the filter, whereas the rings sink.
You'd do well to get some plastic plants - something that will reach to the surface of the water. This will give the new turtles something to rest on when they get tired of swimming and something to hide in.
You'll probably also want something decorative to add interest (your preference of course). A few river rocks on the bottom give the turtles something to fool with and push around and also add interest. If the turtles are small, a coffee cup or piece of big PVC pipe to swim through would be nice. Just be very sure that whatever you put in the tank for decoration doesn't have any holes or openings that a turtle - or turtle body part like a head - could get stuck in. You might like one of the decorative backings that are available for aquariums, too.
You might like a timer for the light. I got mine from lowe's for about $5.
A little fish net, turkey baster, and/or battery powered gravel vac for cleaning up the bottom of the tank is nice. The filter cleans the water but won't really pick up junk off the bottom.
You'll want to glue the intake seive on the filter to the intake tube with aquarium silicon. There have been some accidents on here recently where the seive has fallen off the intake tube of XPs and little turtles have been caught in the current and hurt.
You'll want a cuttlebone to provide additional calcium and to condition the turtles' beaks. Cuttlebone is cheaper than artificial calcium substitutes, is widely available and is very good for turtles.
You're pretty smart to be planning all this before you get your turtle. Most of us learn what we need the hard way.
