I may be getting this wrong but I'm a little confused by those first two sentences.
Let me try to break it down:
If you got a 6670 (or anything lower) you'd be able to crossfire that GPU with the processor's graphics. If you get a second card in the future and pop it in the other PCI-e, let's say a 6770, you'll be able to crossfire the 6670 and 6770, but don't expect 3-way crossfire using the integrated graphics.
I really don't know about the HDD though. If you get one (and that's all it takes) that runs at the advertised speed with the advertised size then I'd say you're good. If it's bad, you'll have 30 days to RMA it and/or the 2 year warranty. 4 eggs isn't bad with 190 reviews though. I've even bought a lot of stuff with really crappy reviews just because of the price/specs and been more than happy with it. I guess it just takes that one dud (which all companys will end up having) for somebody to post an angry review.
So what do you have so far? Are you talking about not getting a GPU at all, or just the second one (which I didn't expect yet)
I really can't speak for desktops on the integrated graphics alone buy my laptop has a single A8-3500m with no dedicated GPU. If you're running just the APU you can expect low settings at descent frame rates in most games, with others like BF3 unplayable. I appreciate my laptop b/c I can play Steam games online at work and stuff but I wouldn't consider it a "gaming" computer.