I used to mock the powermeter crowd, so of course I joined them. I'm on target now to slip below twenty minute on a popular local mountain ride. This is how I've done it.
- Garmin + HRM + Strap/sensor. Using the Garmin's calorie count, and counting everything I ate like a hawk. Lost 30 lbs over a year, back to being a stick. Tons of speed and if you want to race, you'll have to do this anyway.
- Next, I started doing intervals outside. Mostly climbing at threshold, but this was all HR based.
- Sufferfest videos, daily riding (intervals, then easy days) almost every day. Super hard efforts, I take a day off. I've been careful to recover. Four months of this regiment instead of the outdoor intervals made a huge difference.
- Then I got some new books and ponyed up for a Quarq.
Before I got to the last point, I felt like I had hit a new level. Everything was still hard, but my hard was much harder than many of the folks I had been riding with. That's really what I'd stress. A powermeter really will make you stronger. Remember that your heart rate is slow to respond and can climb when you're holding a given load power wise.
But I made so much progress without one that I do feel like you should do all of that first. I will be honest, sticking with hard training videos and really pushing myself on the trainer has had a great effect. Maybe almost as much as just trimming back down. If you're diligent and willing to put yourself through the paces, you can really control your interval training very effectively on a turbo trainer.
Power is the next level. You realize exactly what your weaknesses are. Naturally, when I'm not drinking my way through weekends and gorging myself on pizza/burger/ice cream (I call it the hedonistic Jan diet... which is how I got fat) I'm actually a pretty tiny/light person. So I thought I was pushing pretty hard just because I could drop lots of folks on hard climbs, but after getting a power meter and doing FTP, LT, etc tests... I can do so much better. I would never have known that without a power meter.
Long story short: do all the stuff you know about first for 8-12 months. If you stick with it, then buy a powermeter. There's no point in getting one until you feel like you've really done all you can otherwise.