i believe crutchfield has the OEM speaker sizes in a big list. Find out what the WRX oem size is and go from there. Infinity Kappa's are very very nice, but are a little expensive. before you buy from crutch, always check out ebay. I got 4 speakers for my old 240sx for $100 and thats how much they cost normally for each pair. After that, you should be in good shape. Get some good, heavy gauge wiring as well. thats as far as i would take it anyway, I'm not sure how much boom-boom you like.