I'd say if a car didnt have a sway bar and stock upper strut bar, then those would be the two things i'd get first.
The upper strut bar wasn't all that much of an improvement over the stock bar but when i was busy getting my header installed i took off my road-race upper strut bar and noticed that i couldnt turn with the same speeds i took when i had the bar in. Not that i couldnt actually turn but i felt a tad more unbalanced than I did when I had the strut bar. There's a specific on-ramp i like to hit at a certain speed and basically didnt feel all that comfortable taking it without the bar in place.
The sway bar was the nicest improvement. I noticed the sway bar when taking quick u-turns and canyon runs. In stock form i used to toss the car around just to guage wether or not the road-race parts made a difference. I don't plan on getting rid of any of my suspension parts.