why would you get 275's? why not 280's or even 285's. they still go for around ~200.
two 470's = ~800
i think your pushing more than you can handle. stick to one card unless you want to sli or CF
CF : 5770's CF
SLI: 280's SLI
best buck for your gaming performance unless you have 1k to kill and might need a new PSU