I have a set of these in 19 inch they are replicas of the pegasus wheels fitted to the S60R they are a common wheel choice for modified volvos. Most people would try to steer you away from 20s. Volvo drivers wheel choices are usually limited to larger late model factory wheels.
http://www.ebay.com.au/itm/ALL-KIND-OF-VOLVO-Alloy-Wheel-18x8J-ET43-5x108-MATT-GREY-4-Pcs-Rim-/111561207633?pt=AU_Car_Parts_Accessories&hash=item19f990ef51
Not many volvo drivers are throwing 20s on their cars as you have limited options with the factory pcd of 5x108 and a limit of around 8 inches in width.
this is taken from the ipd wheel fitment guide:
2001-2007 S60 can accept up up to a 235 section width tire a, common upgrades include 235-45-17", 235-40-18" and 235-35-19"