I would upgrade the 650 personally. As a similar person (don't have a lot of money, school, etc.), I find that having a ski that is fun and you slowly upgrade is better than saving forever and not having a good ski. I have a 650sx, and people (like everyone on this forum) goes and gives
advice about buy a superjet. Well news flash, the CHEAPEST superjet around here is $2500. And that's like bone stock, and a '98. That's assuming I can even find one. I would rather take the money from my pay check, and upgrade my 650sx which is a ton of fun to work on and ride, then once I graduate and get some money, I'll keep the my trusty 650sx and go big and buy a total bob-bitchin machine.
Now, I can't give advice on porting or sleeper kits or whatever, I just am in the same boat as you (and as you said, if you wanna keep the 650, why change?). Granted, winter is coming so little or no riding, but I would personally keep the 650 (which again, where I live in CO, a 650 that doesn't run AT ALL and looks like
goes for $300, a good condition one sells for $1000-$1500, so don't sell it for $300 like what's his face said); upgrade the 650, have a lot of fun, once your life isn't a cluster and you got some cash, buy a new ski and keep the current one, or ditch the current one (I love my 650sx and put too much work into it to sell it, even once I buy the fanciest jet ski ever).
Don't know if that helps. Good luck bro.