Firstly to answer Joyce question I refered the Mark Cade book, the example assignment is good but I only used it as pointers as to how much detail to go in etc. Mostly I depend on my experience.
Secondly to answer Yifei Hong Question I have been in development from 8 yrs, and been into design and architect work for last 2 years, But there are lots of difference between real life project and this dummy assignment coz its not the full product its a subset of a full project. Anyway since I have been developing in J2EE from last 4 years and luckly some of my projects required me to use third party software/external system which helped in design of the assignment.
Now to answer whether you are in the right position to shoot for SCEA? I don't know as only you know how much you are capable of coz I know guys who are working as designer/architect but will not pass this exam, but also I know people who are just team leader / senior developer but have to potential to clear this exam easily.
Now the important question how which you might be interested in a comparision of SCEA with SCBCD / SCWCD. IMO this is what I think others may not agree with me.
SCEA : Good for design and architect people i.e. do you need to design or architect projects in your current work? Does your company wants only to develop in J2EE b'coz the whole exam is centred on it it will only help you become a Sun J2EE Architect and not a general architect, you will have to learn more stuff about other architecturs such as .NET etc. You will not gain any hands on (coding wise) after doing this exam, the SCEA part one books are very boring IMO. So to wind up if you are a developer and see that in near future you might be promoted to Architect and will do 0% coding then go for SCEA but if you will have to design and code as most of the developers in small company do then you might first do SCBCD/SCWCD and then just go thru the SCEA part one book and see if that what u what to do coz IMO the exam cost is too high.
SCBCD : Good if you are a developer it will give you a good indepth knowledge about J2EE and hence will be very helpfull in development and here you will learn design patterns not only how and when to use but also how to use coz in SCEA you will only learn when to use and not how to use.
SCWCD : Good if you intereseted in front end development me personaly not so interested in front end development I like EJB's but its own personal choice again this exam contains design patterns which are usefull for front end developments and yes you will learn how to use it and not only when to use.
Hope my points will help you all let me know if you need more info.