if the number of sequences is not too big, you can save and open the following HTML page (it uses the HTML5 file-API to load the data):
EDIT:: i've updated my code. It handles multiple FASTA files.
Easy command line tool (faCount) from UCSC (download page for linux- http://hgdownload.soe.ucsc.edu/admin/exe/linux.x86_64/) will give you nucleotide composition of each sequence or can give you a summary of total nt content for the input fasta.
(faSize is another handy one to summarize the number of reads and total number of nt)