IBM's passport advantage is the standard download place, if you have a password.
WebSphere is JSR168 compliant, so you can develop your portlets anywhere, using any tool, but IBM's Rational Application Developer plugs into the WebSphere Portal Server the best.
As far as XP goes, it's fine for development and testing. People ususally use server software in production, but I've got the whole thing installed locally on my Windows XP box running on a Dell with a gig of RAM.
I have some free portal development tutorials if you'd like to check them out. They demonstrate how to create standard JSR168 compliant portlets with IRAD.
Indeed Lalit, the site seems to be confusing..but I believe you can download WPS5.1/6.0 only when you have IBM Passport Advantage / Express. For IBM ID only IRAD is available without WPS. One more thing if you can get hold of Rational Software Architect 6, it has standalone WPS5.1 installable bin there but I could not find that either.
Let me know if you find out that..I am also in same boat.
A good portion of the IBM Test 399 and Portal test 829 has to do with development. All of the correct development answers have to do with JSR168, although I did see some references to the JetSpeed PortletData as some incorrect options.
Some of the extra WebSphere Portal Server specific stuff has to do with skins and themes, namely the various wps custom tags you can use in a skin.
There were also a few questions on the IBM Struts Portlet, which is a little different from the Apache Struts Portlet Bridge.
There is probably one question on the WebSphere Portlet Catalog, a question or two on the personalization engine, and a little on WebSphere configuration, where you need to know a little about XMLAccess exports.
So yes, to really kill the certification, you need to know WebSphere Portal Server (WPS), but having said that, developing on JetSpeed2 and learning the JSR-168 API is not a bad way to start.