Yeah that internet speed really isn't that good for streaming, mine is barely better than that. (20mbs down and maybe 2.0mbs if I'm lucky Upload)
You could at a lowered resolution, I'm sure it might be good enough for at least 480p streaming, as a viewer I'd be fine with watching somebody on 480p, just means it would eat up my internet alot less than somebody trying to do 720p, or heaven forbid somebody trying to do 1080p lol.
In that case either would still be good, but the i5 would likely appear fast in day to day stuff due to the much stronger cores, the 280x is a really good GPU. I recommend an SSD as the next thing you ever upgrade, it's a night and day difference, at least to me, and I use my pc alot per day. I used a HDD for about 8 months and it was still good, just sometimes sluggish.
I could of honestly went AMD and probably been just as happy, I don't play stuff like RTS or MMO super tryhard competitively where the better single thread performance would matter. Mostly just console ports cheap or whatever pc stuff is cheap on steam sales.