I think you should take the SCJP 5.0 certification exam because it is the latest one available.
I've done SCJP 1.4 and I can say that many companies don't care about the version of the certification, they care only if you have one. And I think many companies still use jdk1.3.
But as you are going to spend energy and time to prepare for it, you should try SCJP 5.0. Sooner or later a new Java release will appear, so you're certified on the last version.
That's my vision.
SCEA 5 (part 1), SCBCD, SCWCD, SCJP, CLP, CLS
Joined: Jan 02, 2007
may I ask you a question first? why do you want/need to take the certification? I am studying java because it's five years I work in IT fields, and my knowledge cover several fields, but nothing really deep. I choose to expand my knowledge in java because it's one of the leading languages, and my company is aware of it. Personally I don't see any reasons in studying an obsolete version of java virtual machine. So I go for java5. The program (objectives) are a bit wider of the former, so a bit more difficult. But it's also more modern and will last on the market longer. And someone says java 6 is at the door...