My 99 S80 was rattling and clunking over every anomaly in the road (87K). I assumed I needed the strut mounts so I ordered all the parts (mounts, boots, bushings (aka spring seats), bumper stops and struts), about $350 in parts. When I got under the car I found the struts and and all related was tight and in good order, but I noticed one of the boots on the sway bar link was torn. I replaced the links (about $40 for both) and it was as quiet and tight as ever.

Not really suggesting that that's what you need to do, but just mentioning it for other readers who might have a similar problem, and an easy something you might to replace while your at it.

So, now I have everything waiting for when the struts do go. I got the Monroe struts at about $88 each. I know, you get what you pay for, but I've had okay luck with Monroe's and I'm not looking set my car up for a road rally. Assuming your set on doing this, obviously if it were me, as I've just demonstrated, at that milage I'd replace it all. You should get new bolts for the strut-to-knuckle mountings and the six strut mount nuts as well.

Make sure you have a good solid spring compressor to change out the struts. Very dangerous to use crappy, cheap tools for this job.