I have a fender frontman 212 Dsp, and i was wondering since i can get a cab for it, if i should buy a cab for it, then when i get the money, sell the frontman and buy a nice tube head? I know a cab would increase the sound quality/ reduce feedback.. so.. would this be a good idea?
A cab wouldn't increase the sound quality or reduce feedback. Basically I'm saying no. Save up for a good amp, sell the frontman and put it towards your fund and buy a good amp.
I don't give a shit if you listen to me or not
Good point Kevin. So, are you saying that a line 6 valve halfstack say 100 watts, would sound the same as a spider valve 100 watt combo?
Not exactly, but close. The real difference would come from the combo being open and the stack version having a closed cab.

An extension cab is not necessarily a bad investment when you have a nice amp, but if you're trying to play metal with that frontman you're in desperate need of a new amp and that should be the number one priority on your gas list, not a metal muff.
I don't give a shit if you listen to me or not
If you want to get a cheap fix, try the Digitech Grunge pedal on the metal setting. I own that amp and pedal, and I with them I got a fairly decent tone. Save up for a new amp though, that thing isn't too great for metal. If you get a good amp, you'll be wondering why you paid $100 for a pedal you won't need.

As well, don't turn the volume knob past 2 or 3. :P