As for flash, before you buy anything it might be worth your while to invest some time at strobist.com.
I second this. Off-camera flash, used right, makes more of a difference to the quality of almost any photos than lenses will. A poorly lit shot will look just as bad on the nicest lens you own as the kit lens made of plastic and tissue paper... Lighting is often overlooked, but critical to the end result.
That said, if you're doing macro work, you might well find you need the macro lens, if you're shooting on safari, you'll need a telephoto - but a flash will be usable in almost any 'regular' situation.
Personally I went for an E-TTL flash for my Canon, but if you're doing things the 'strobist' way it's not necessary at all. A $50 flash from eBay works every bit as well!
To echo what everyone else says - look at what you're shooting, see what you enjoy most, and ask yourself honestly what you
need to make those photos better. I spend half my non-photography time talking myself
out of the next 'essential' bit of kit, as I simply don't need it.
Mark