Do you really have Summer tires on there right now? Or are they all seasons?
Since it sounds like cost is a big factor, the way I would go about it, is Tires-->OFT-->Header-->Catback-->Flex Fuel Kit.
When I started modding my car, I lived in Arizona so i actually got OFT & Header at the same time instead of tires. However I grew up in Chicago so I know how important good tires can be. I've missed class before because my brand new FWD Civic couldn't go up a slight incline with snow
The catback requires the least work to get going which is why a lot of people get it first, but it is the least effective if you are looking for a little more hp.
Believe me though, while E85 & Header will make the car feel GREAT especially if done at the same time, you will still not be beating anyone around town, it's still only 190-200whp. The car would be great for track & autocross events though =)
And personally, the reason I would save flex fuel kit for last is that I have pumps very close to me with E85 (and all are at least 70% ethanol) so I rarely have to change out. But, if I have to switch back to regular gas, the OFT allows you to save up to 5 tunes on it which can be uploaded to your car in about 10 minutes (find the threads on here on how to do it).
I currently keep the v4 Gas, v4 E85, v4 Gas + Pops, and two v2 tunes because i don't have anything else to keep.