Well, first things first, what point level are you playing at and working towards?
Secondly, grab yourself some more Transport vehicles when you have the cash; the current preference being Venoms over Raiders. Transports are crucial to our army.
After that, grab an HQ like Archon or Succubus; pref being the CC oriented Succ.
Against Tyranids, I would go gun boat and mow down the bugs as they come at you, but this depends on the type of army they are fielding.
For killin MEQ's, I would suggest the same thing but with some good lance backup to knock out their vehicle support.
Any advice past this would be point size dependent and hing on you getting a couple more models.