I'd say not worth it. The 6450 is beyond low end, mostly for a dual monitor setup on a budget, those APU's have pretty killer graphics on their own, i'd say either go with an FX chip and get a discrete card for all your graphics needs, or throw all your eggs in the APU basket and save that money for some other fun upgrade. I run AMD CPU's and Graphics, but i must say that the crossfire stuff doesn't work super well in most cases in my experience, which is what you'd be doing.
Oh I already have the 6450 as an old card. I was seeing if it could give me a small fps boost until I can afford a 7750.
Ah, i'm pretty sure it's possible, so you should see minor increases in something like FPS. But with crossfire there are lots of microstutter and dropped frame issues (with the current drivers at least) that might result in a net loss of practical performance. Your call, it should work so it wouldn't hurt anything to try it.