To capture a tcpdump that’s readable from wireshark do the following:
tcpdump -i <interface> -s 65535 -w <some-file>.cap
Where <interface> is coming from an ifconfig, and <some-file>.cap is your capture file name, i.e. tcpdump_09242012.cap.
Then just transfer to a computer with wireshark. *.cap should already be a filetype assigned to wireshark so you can just double click it.
I’ve successfully done this tcpdump on a RHEL5 system.