Originally Posted by ZipSpeed
If you're crunching projects that Radeons excel at, it might be worth it to jump to a 7950 to get the better double precision throughput.
to elaborate on that, the HD 78xx GPUs are FP64 (double precision) capable, but their FP64 performance is abysmal. 2 generations ago, Evergreen (the HD 5xxx series) implemented a VLIW5 architecture, which allowed FP64 calculations to operate at 1/5 the rate of FP32 (single precision) calculations on FP64-enabled GPUs. 1 generation ago, Northern Islands (the HD 6xxx series) implemented a VLIW4 architecture, which allowed FP64 calculations to operate at 1/4 the rate of FP32 (single precision) calculations on FP64-enabled GPUs. with the current Southern Islands generation (the HD 7xxx series), AMD decided to split its FP64-enabled GPUs into 2 groups - those who's FP64 performance can achieve 1/4 of their FP32 performance (HD 79xx cards), and those who's FP64 performance is limited to only 1/16 of their FP32 performance (HD 78xx cards).
obviously this matters if you intend to jump right back into Milkyway@Home since its one of the only DC projects that requires double precision calculations. take a look at THIS PAGE OF CHARTS
and you'll see that despite improvements clock speed, bandwidth, etc., even an HD 7870 GHz Edition can only achieve 160 GFLOPS of FP64 performance. your HD 5850 was capable of 418 GFLOPS of FP64 performance just to give you an idea of how much of a step backwards you'd be taking by investing in the best 7870 out there, let alone a lower 7870 or 7850. of course if you aren't going back to MW@H and are only concerned w/ FP32 (single precision) performance from here on out, then none of the above applies, and i would recommend a 78xx series GPU over a 6xxx series card or 5xxx series card from both a performance and power consumption perspective.
*EDIT* - just realized that you don't really want to spend more than $200. even after rebates its going to be tough to get an HD 7950 for less than $260 right now (unless you buy used - i got mine used for $240 shipped). that said, a used reference model HD 6950 will only run you $130-140, and it will have approx. 78.5% of the performance of an HD 7950, while consuming no more power than an HD 7950. for ~$150 you can probably find yourself a shader-unlocked HD 6950, which will consume marginally more power than a reference HD 6950 (and thus an HD 7950) due to the extra unlocked shaders. OC it to 880MHz and you'll essentially have yourself an HD 6970, and it'll have approx. 94% of the performance of an HD 7950, but the extra power consumption will be non-negligible...
1) X6 1090T, Hyper 212 EVO, ASUS M4A89GTD, GTX 580, SeaSonic X-650, Win7 x64
2) X6 1090T, Hyper 212+, MSI 790FX-GD70, 2 x GTX 580, GTX 560 Ti, SeaSonic X-750, Win7 x64
3) X4 1055T, Hyper 212+, GA-890GPA-UD3H, HD 7950, HD 6970, SeaSonic X-750, Win7 x64