Can anybody tell me the best way to modify my 03 wrx for more power

 

either PPP through your dealer or Ecutek and exhaust system if warranty isn't important to you.

 

As above, use search for PPP related threads. But as a summary
PPP is warranty, resale and insurance friendly
ECUtek(and others) provides a better basis should you want to go for more power than the PPP provides, but is you'll get less on resale, will cost more in insurance and will void the warranty.

 

I've got a my03 wrx 5dr with ppp, used to drive a my02 sti, the wrx is just as quick as the sti, handlings just not quite the same. It keep's up with my mate's sti, i'de recommend sticking with the ppp and keeping your warranty.

 

Or try this http://www.wrc-tech.co.uk/subaru-04sti-WRC320.htm they do a waranty as well