My 2005 Magna wagon has OK radio, it's the std. one, but it's nowhere near as good as the std radio in my wife's AUIII futura.

Any recommendations for improvement? Better headunit? Speakers? I don't want to spend a fortune.

Well, it depends on the definition of a fortune!
The Pioneer DEH-3100UB or DEH-4150UB are good choices, great sound, radio reception (if you use radio), USB connection, direct sub drive (allows you to output the rear speakers as 2x50 sub channels for your 6x9's. etc

The last feature is good if you want some bass but don't want to shake your car apart or damage your hearing too much. But you need good rear speakers for that :)

AFAIK they don't support digital radio though, and I'd be very cautious of getting models from overseas that claim they are digital radio upgradeable. Australia uses the DAB plus system, its probably the best system but is also the least common worldwide. There are several different types of incompatible digital systems worldwide!