It really depends what you mean by significant? Reading between the lines, it seems as though you want to try to separate 'real' contigs from assembly artefacts. If that's the case, you should think carefully before discarding transcripts with a low RPKM.
There is no minimum - a contig representing a real transcript can have very low numbers of reads mapping to it, and have an extremely low RPKM. Equally, a high RPKM doesn't guarantee that the contig represents a real transcript. We often see chimeric contigs - where fragments from two or more different transcripts have been assembled into one contig. These chimeras often have high RPKM values, even though they are artefacts.
So, the answer to your question 1 is no, high RPKM does not mean you can be confident in the transcript - it isn't reliable. Thus there is no appropriate RPKM to making such a decision.
As for question 2, it really depends what you want to do with your assembled transcripts. Are you performing differential expression? Motif discovery? Are you interested in a particular set of genes?