WebParts is now part of ASP.Net 2.0 Framework and part of System.Web.UI.WebControls
and now it become System.Web.UI.WebControls.WebParts.WebPart
Also override method RendorWebPart is now Changed to Render
Develop your webpart give strong name deploy it in GAC.
Add assembly information in Web.Config in SafeControl Tag and reset th IIS.
Go to Site Settings -> Site Collection Administration ->Galleries click WebPart.
select webpart and click Populate Gallery