stick to your budget. but do a better job shopping around, including looking at lightly used bikes.
Yeah, you may upgrade later if you become a bike fiend. But you may not. Plenty of people out there selling 'lightly used' bikes, after all. And if you're a new rider, you may not really appreciate the difference between them anyway.
The 'buy for the future' argument is sort of silly. How do you know what you'll want in the future when you aren't riding yet? For all you know, you might want a ROAD bike--in which case, better buy a cheap MB now. And so on. That's just a concept which salesmen use to get you to go over your budget. It makes sense if your budget is $300, as any serious riding and you may be wishing you had upgraded. But $800? Please. you can get a VERY VERY nice MB for that money, and if you're incredibly unhappy with it after a year, you either got very good very fast, or you're a total gearhead.
Hell, go buy a Motobecane off bikesdirect if you're so worried... you can swap the components to another frame later if you want to and make upgrading cheaper.
though I'll say it: why on earth do you want a DS with discs?