not a fan of the frame, but i'd go for the fork. but if you look around, SID's are going for as low as $50-100 if you're lucky. why would you want to accumulate an extra frame? but $250 for frame and fork isn't necessarily a bad price, but consider by the time you build that frame up, you'd spend a lot of money on a basic trek 4500 ($300 bike new).

anyway, why are you trying to upgrade your bike? is it not serving you well enough? i mean what exactly is wrong with it? i can imagine the fork needing replacement, but other than that, is there something troubling you with the drivetrain? v-brakes would be a good addition, if you must upgrade something. then look on mtbr or craigslist.org to find a cheap used (decent) fork.