IIRC, 2003 had different tophats for the rear springs than the STis. To use a STi spring you will need either the STi tophats or an aftermarket one designed for the STi.
I know you are on a budget, but if you are buying new parts you are looking at around $500 for a set of KYB G2s, $250-300 for the STi springs, and I'm not sure how much for the top-hats. Of course if you find all this stuff used you can get it cheaper, but if you are buying new why not pay slightly more for the coils. BC Racing BR Type Coils retail for $999.00. I can probably get them in for you for less than that.
If you were just swapping springs that's a lot cheaper than coils, but if you have to get struts too then the price difference is negligible and well worth it for the coils. Plus you won't have to assemble them either

-Aaron- So called "Ninja Lawyer"
2003 WRB WRX
2007 UGM STi
Need an improvement in your health, energy levels and/or bottom line? Ask me about Vemma/Verve. It's changed my life.
http://ahargis.vemma.com/