Sounds like a fair deal to me. I work two different jobs(both part time, one makes the money, one is for fun). The fun job is working at the bike shop.
For an aluminum frame bike, we start at $370. $370 aluminum bike is good quality, but not as rigid as a more expensive double-butted mountain frame($550+).
Little fact: stx-rc is no longer in production. Was last in production several years ago. Some models still come equipped with it, since companies have so much backstock. Many shops still have stx(and rc) parts in back(we have a few left), but are disapearing. If parts break(most people catch a log on the rear der) you will either have to go higher up in Shimano's line-up(LX, XT, XTR)or lower(Alivio, SIS, etc).
Good call on the Giant frame. For those not in the know, Giant frames are used by many companies. Some companies use their own(Specialized, Cannondale, to name a couple), but many use Giant frames.