I download my SCJD assignment about a week after passing SCJP. The normal way to go would be SCJP,SCJD and SCEA. But since SCEA does not have any pre-requisities, you can directly go for SCEA. However unless you have some job experience working with Java, design patterns etc, you should do some reading before attempting SCEA. Also you will need to know quite a lot of stuff for SCJD. You will need to know good design patterns, Java coding standards. Among the technologies you will have to know RMI or Socket programming, Swing, IO related class usage etc. IMHO (In my humble opinion), you should first go for SCJD rather than SCEA. Also since there is no time limit to complete the SCJD assignment, there is no harm in downloading the assignment early and taking a look at it to understand the scope of the assignment and get a feel of the real thing. Also download it only if you are serious about completing SCJD, as it does cost quite a bit of money 250 USD.
Hope this helps . . .
All the best for your SCJD or SCEA.