...umm...upper engine mount? This is the one that is often a problem with 850's and S70's. IPD makes a polyurethane piece to replace the original one, (http://www.ipdusa.com/ProductsCat.aspx?CategoryID=256&NodeID=4720&RootID=629) but I've heard that it is very stiff compared to the OEM mount and you can feel more road vibrations through the steering wheel. Many people opt to replace with the original (almost circular) mount. If your problem is the rubber mount, it is not a serious problem if taken care of fairly soon...however, if you keep driving your car with the upper mount broken the lower ones will also break and this could lead to some major transmission issues...but you will know for sure that something is way wrong if the engine rocks excessively back and forth on take-off, when you change gears and come to a stop. It's a cheap and easy DIY job. Just check the top piece...just about in front of the driver's seat under the hood...you'll see a metal ring with a rubber piece inside of it and a long bolt through the whole contraption. Visually inspect it for the broken rubber bushing coming apart from the metal ring, and if it seems okay, see also if you can move the mount housing. The rubber mount should be tight in the housing and should not allow any movement.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon