Champion, if you don't get at least 320, then you'll fail, and to be able to submit your assignment again, you'll have to acquire another voucher. When you apply for this certification, Sun will send you one of the assignments. There are currently only 2 types of assignment: Bodgitt & Scarper and the URLyBird. Each one has several versions, but their essence is pretty much the same.
After submitting your project, you'll have to take an essay exam, which has 4 generic questions. The objective of this exam is just to verify if you were really the one who developed everything. The questions are about what you used, how you developed things, and so on.
But the good news is that we are here to help. There are good resources/tools around here, and people here are very friendly! So no need to worry and whenever you need help, please let us know, partner!
Just a little expansion on Roberto's comment on resubmission - there is a separate Assignment Resubmission option for anyone who needs it. This is much cheaper than the original assignment, and ensures that all you need do is fix whatever you failed in within your current assignment - you do not need to start a brand new assignment. There is no need to re-take the essay exam.
But all of this should be unnecessary. The members of this forum give great advice, so you should pass easily on your fast attempt.