Personally I say get a whole new complete bike!
I work in a shop...and I like to build up my own bikes cause I get everything so cheap! But even then, it's better to buy a complete bike. I'll give you an example... I have an old specialized hardtail I want to get up and running again. I need new cranks and a fork for it...oh a front derailure too. Well I started pricing everything... and just for the hell of it I priced out a new HT from specialized...nothing fancy just descent.... Well it turns out I can get a whole bike for about 50 bucks more than what the parts to build up my HT would cost. Granted they aren't all the highest end stuff and not the same EXACT parts I was looking at...but it will get me up and running!
In short...for what you'll spend to upgrade and get your bike back up and running you COULD spend almost as much as you would for a new bike!
But of course this is all depends on what you want to upgrade and what you're looking for. With the newer bikes you will have the disc tab on the frame so if you wanted to ever go with them it would be no problem...unlike your old bike.
Just some things to think about...hope it helps!
