Not an expert on clay soil but i know it can be very hard to grow anything in it. You could try digging in some sand and manure, to encourage nutrients plus the sand will encourage drainage. If you can get a good mixture then basically you should be able to grow anything, but there are certain plants that could handle clay soils better...
Most normal perrenials will grow ok perrenial Greaniums Ladys Mantle Lupins Hostas Columbines etc I think its just a case of trying to encourage drainage before you start and making sure it does'nt dry out if we have a dry summer (remember them)