This is what i'd buy on a $800 budget :
$20 Any piece of crap case
$100 Any Z87 motherboard
$50 Any half decent brand 600+W PSU with two 6-pins
$200 Haswell i5-4570K
$25 Hyper 212 aircooler - Squeeze that K series CPU hard
$280 CF HD7850 on par with Titan if squeezed fairly hard
$75 2X4 GB...
Mine is locked at 1.30V & 4.5Ghz it idles at 29°c and gets up to 75°c on intel burn test, about 55°c gaming.
Aircooler is 412S push/pull config. I could go lower voltage but I see no reason for that, voltage and temp is acceptable and at that voltage it's always going to be rockstable.
The jump...
>€20 Random cheap case
>€70 Athlon II X4 750K (four cores, overclock to 4.2 - 4.5Ghz if you won the sillicon lottery, doesn't bottleneck a highly overclocked 7850)
>€45 asrock board
>€135 MSI 2GB HD7850 OC edition (overclock to at least 1100mhz core, I got two in...