You don't need to do SCJP before SCJA
One thing I've seen is people doing SCJP on 1.4, which is an easier exam than 1.5, with a lower passing score (52%).
Then, you get your SCJA on Java 1.5, which is the only version it is currently out on. Then you can brag about being certified by Sun on both Java 1.4 and Java 1.5. It looks great on a resume, especially if you're looking for a Java job.
As far as SCJA mock exams go, or even some free tutorials on how to get Sun Java Certified, check out the following links:
http://studyguides.scja.com/ExamScam/sunjavacertifiedtutorialsandmockexams.jsp?link=mockexams
http://studyguides.scja.com/jsr168/ibmportalcertexam399.jsp?link=mockportalexamsmockscjaexams
Enjoy!
-Cameron McKenzie
http://www.scja.com
http://ww.portorials.com
http://www.scwcd.com