I think thread counts can go up to 7-800 and the higher the count te softer the sheet. But do they last longer? I hear buying organic cotton sheets that are at 320 thread count are better than 700 of regular cotton?
Egyptian cotton is meant to be the best cotton in the world. Expensive yes, but long lasting and worth the money. I have a colleague who bangs on about her Egyptian cotton bedsheets, apparently even after 2 years of using them, they come out like brand new after every wash :)