The rears are easier. Just undo the lower damper nut (21mm) and slide the bolt out partially. Undo the spring lower cup nut (10mm) and remove the bolt (otherwise the spring may dislocate itselt at the top and damage the bumpstop). This bolt will break and a new M6 nut and bolt will be neeeded. Then when the spring is fully extended, there is a plastic cap in the chassis leg which conceals the 15mm top bolt. Take care when removing this though as if the bolt slips, it will fall down inside the chassis leg (a telescopic magnet is invaluable to retreive the bolt).
The front struts are quite costly. I ended up using Monroe OE replacements (£60 each-trade) but obtained a couple of nearly new Bilsteins for the rear. I can't tell you how good the Billys are- just better in every way. The car leans less with them, fast cornering is less likely to bring on roll oversteer, adhesion is better- especially over rough surfaces, and ride quality is also actually better due to reduced body motion.
The Monroes at the front are similar to OE, but damper companies claim their replacement dampers are about 15% uprated (to compensate for suspension wear). I can't really say just how they campare like for like though.
Remember to get new damper protection gaiters/bump stops at the front if necessary. Monroe now market a protection kit for the front of the 155 (cost a little over £20).
Also beware of replacement damper companies who market a very rationalised range- their dampers may not be specifically valved for the sporting 155, but more likely have a comfort setting for Fiat Tipo and Bravas. Boge, Magnetti Marelli and Unipart fall into this catagory.
BTW, the lower front strut bolts are 15mm heads and 18mm nuts I think.