Hi, just bought a rb450G with RouterOS. Is there anyway I can connect to it without using a null modem cable.

Download Winbox from MT site
http://www.mikrotik.com/download/winbox.exe
Connect your RB450G via Ethernet port, run Winbox, press button with three dots.
After few seconds you will see RB450G on list, select MAC address and press Connect.
User is 'admin' without quotes and no password.

And from linux?

Use Wine, Winbox run on it.