Since you're an English bloke, there's only one thing you need to know about upgrading wheels - Paul Hewitt (at Paul Hewitt cycles, funnily enough). An unbelievably good wheelbuilder (pro team, national team, olympic teams yada yada yada, been building for decades) that can build wheels to suit your needs given your weight and type of riding. He's a fun guy to talk too as well... so many stories about funny cycling sh1t that you could be there all weekend and he'd still be talking. All of the wheels that Paul built for me have remained arrow straight - even the really lightweight ones (mavic gel280s - 28 spoke) ridden on flat tubulars on rough roads up in t' hills.
But, hell hath frozen over and for once I agree with Tony Z...
A power meter should help you train better and therefore should allow you to go faster over time. Of course you still have to go out and pedal and you still have to come up with a good training plan.
A few benefits from using a PM too:
-Better pacing in time trials
-Slightly easier to test if a new position is faster
-Downloading the data onto a PC creates the mother of all brutally honest training diaries
-Test whether pacing and feeding stategies work sufficiently in longer events.
I wouldn't think too much about the "my bike is only worth 'x' amount - how much should I spend" because stuff like wheels and power meters can easily be moved to the next bike should you ever change your ride.
Use the power meter to help with training then if needs be a flash set of wheels is only a visit to the bike store (or a phonecall to a friend to blagg a set for the weekend) away.
If you go for a PowerTap wheel - I'm sure Paul could build one of those :P