7970
Mmm maybe.
7770: ~200Mh/s @ $105 = ~
1.9MHs/$
7850: ~300MH/s @ $160 = ~
1.9MHs/$
7870: ~400MH/s @ $220 = ~
1.8MHs/$
7950: ~500MH/s @ $290 = ~
1.7MHs/$
7970: ~650MH/s @ $400 = ~
1.6MHs/$
A major factor is your side costs. If you've got a budget of $3,000, you would only be able to buy ~20x 7770s, as this would require 3-4 computers to run them. 4 motherboards, 4 CPUs, 4 PSUs, 20 risers, etc. all add up very quickly. You might be more interested in buying 6 7970s, and fitting them all only one motherboard, with one giant PSU. You're looking at one computer rathen than 4, and roughly the same speed.
The other thing is deals. Obv, if you can find a 7950 for $220 rather than $290, than it's more profitable.
My end suggestion: Plan on building one rig: 1 MB, 1 CPU, 1 RAM, 6 risers, and everything else. Then take your remaining budget and split it up between buying the fastest 6 cards you can afford, and a PSU big enough to power them. If you've only got a $800 budget, then get 6x 7770s, and a 700W PSU. For a $1,200 budget, then you might be looking at 6x 7850 and a 1000W PSU. If it really is $3,000, then go for the 7970s and a nice 1400W PSU.