I'd go with speakers first. On my '04 EX, I got a new head unit first, but it still sounded like crap because of the stock speakers. I got Infinity Reference 6.5" components for the front and they sound great!

I would go with those as the first upgrade so you get good clear sound, then when you get some more cash, upgrade your head unit to something like the Alpine CDA-9807. This thing ROCKS, plays MP3 and WMA, and overall kicks major booty.