The most efficient cards in terms of points per Watt are the GTX580 and GTX570.

One GTX570 in one system would cost much less than two than 2 systems each with one GTX460, and do more work. It would also save you on power.

If you cannot aford a GTX570, then get a GTX470.

If the system is just for crunching you should be thinking about using Linux x64 or WinXP. Don't use Vista or Win7.
If you intend to use the CPU to crunch with elsewhere then remember it is more efficient to crunch on a GPU and to optimize here you need to leave one CPU core/thred free.

While the i3 and i5 CPUs are reasonably efficient ~ 73W, there are less expensive AMD options that only use 45W:

AMD Athlon II X3 400e 2.2GHz Socket AM3 L2 1.5MB 45W PIB £65
AMD Athlon II X3 415e 2.5GHz 1.5MB AM3 45W Energy Efficient (Retail) £75
AMD Athlon II X4 600e 2.2GHz Energy Efficient Socket AM3 L2 2MB 45W PIB £90
AMD Phenom II X4 910E 2.6GHz AM3 8MB £130
An AM3 motherboard with one PCIE slot can be picked up for around £30 to £35.
Dual PCIE slot AM3 motherboards start from around £45.
An i3 starts at £80 and use 73W; so they cost more to buy and more to run.

While an i3-530 would probably slightly outperforma an Athlon II X4 600e, in terms of performance per Watt it would lose by some margin.

As for RAM, to crunch here on Linux 2GB DDR3 would be plently, but if you want to crunch some CPU projects have a look at their requirements.

The GTX570 memory interface is the same as the GTX470, which does well enough here, and shader frequencies are very important when it comes to crunching here.
I have no doubt that the GTX570 is a good card for crunching at GPUGRID, especially when compared to a GTX460 in terms of performance per watt and purchase costs; the GTX460 basically performs with only 2/3 operable shaders. The GTX460 is inefficient compared to a GTX470 never mind the more energy efficient GTX570.

In order, the top cards for here are,
GTX580, GTX570, GTX480, GTX470, GTX465.

There is a nice comparison table here:

http://www.hexus.net/content/item.php?item=27782

Real-world performances do not match up evenly with GPUGrid crunching performance. Hence the CC correction factors used to correct GFlops peak performance for actual GPUGrid crunching performance. For example, a GTS250 for normal use totally outperforms a GT240, but not when it comes to crunching; the GT240 is better (faster, more efficient, more reliable and cheaper to buy). Primarily this is because the GT240 is CC1.2 rather than CC1.1. Similarly the CC2.1 cards do not perform as well as the CC2.0 cards.

Yes, GPUGrid can utilize two GPU's in one system; I have two GTX470's in one system (XP), four GT240's in another system (vista) and a GTX260 in another system alongside a GT240 (Linux).

The thing you must check is the power requirements; one GTX570 has a TDP of 219W. I suggest using a 750W (or higher) PSU. Look for a PSU with four 6pin power connectors for the GTX570 GPUs (if you decide to get two). The more efficient PSU can save a lot of electric over the lesser PSU's.

http://www.techradar.com/news/computing-components/upgrades/best-psu-14-pc-power-supplies-tested-908503?artc_pg=2

A GTX570 review by Ryan Smith for AnandTech.

OS: Linux if you're familiar with it and know how to install and handle the nVidia driver - otherwise go with XP.

PSU: definitely 80+ Gold. The electricity is going to cost.

MrS
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Scanning for our furry friends since Jan 2002