The road legal track tyres generally used by people with 944/968's are: Michelin Pilot Sport Cups, Dunlop SSR, Toyo R888 and Kumho V70A (listed with cheapest last). I have used the Michelins and Kumho and found that the Michelins are fantastically grippy in the dry, almost like a full slick, but can be a bit hairy in heavy rain on the road. The Kumhos are better in the wet but not as outright grippy when dry. All trackday tyres are far, far, FAR grippeir than normal road tyres when up to temperature
Camskill have a good website that list most brands and prices
http://www.camskill.co.uk/products.php?plid=m12b0s0p0