Two GPU's ? --- Info needed

I agree with Grant but my experience shows: Go for a Gold 750W PSU or more, give your PSU a chance to work for years, 650W is to close to the edge, it´s works but the question is for whow long? rememeber when you crunching SETI you stress your host components almost to the limit. In the US the price of both will be almost the same.

I never see a cruncher who say, my PSU is to big...
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It is possible to have a PSU rated too powerful for a given system - different loads often exhibit different power efficiency ratings, so if a system isn't drawing much power compared to what a PSU is rated for, then it could be that it's not performing its best in terms of power efficiency. Although, these days, 80 Plus rated PSU are usually fairly consistent in efficiency across varying loads, so it shouldn't be too much of a problem.

For this particular case, 750W probably won't be overkill. But I was just replying in response to the statement 'no-one will say my PSU is too powerful', considering some go way over a kilowatt rating...

At any rate, I usually find this non-vendor specific PSU calculator to be useful guide to buying PSUs.
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Soli Deo Gloria

All my 500 and 650 watt PSUs have died within 2 years. I now only use 750 or 1000 watts.

Either of those cards would be good, as they are the same series as the existing card. The only problem will be if you run more than one WU at a time.
The GTX 460 series can run up to 3 WUs at a time, but you need more than 1GB of Video RAM if you want to use the system for anything else.
With 1GB of Video RAM it can do 2 at a time with no problems.
But with only 768MB of Video RAM you'd only be able to run one at a time- running 2 WUs at a time with an optimised application on my GTX 460 uses 745MB of video RAM with a couple of browser windows open. Do a few more things and it goes over 800MB.

Also what is the MIN PSU needed to run those two cards ?

They're rated at 160W max.
For this sort of work you'd want a minimum of 300W just for the video cards, allow a couple of hundred watts for the system (for the AMD, the P4 would be at least 250+).
No PSU likes to run at 100% load all the time, i generally rate mine so at the absolute maximum load it's at no more than 75% of it's maximum rating.
So that PSU wouldn't be up to the job IMHO. A good 650W PSU would be plenty.

But with only 768MB of Video RAM you'd only be able to run one at a time

You are wrong. I run two WUs at a time per card on my machine with dual GTX460 768MB cards; surfing causes me no problem when doing so. GPU1 (monitor attached) uses 607MB video RAM and GPU2 (no monitor, just CUDA) uses 473MB. When running 2 browser windows also GPU1 goes to 645MB in use.

And I have also watched football games at the same time from NFL.COM with no stuttering or lag. But I wouldn't advise heavy gaming...

Right now my 768Mb gtx460 is running two MB quite happily (if a little on the warm side as it is slightly overclocked and needs to have the current resident dust bunnies evicted from their warren.
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Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

Spitfire, while you may be able to load 4 tasks into memory your GPU is running at 99% load at two, three and four tasks, so each additional tasks beyond two will be taking more time, so isn't worth it in terms of throughput.
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Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

Not entirely true.
A friend is running 3 instances on a 450 and it still benefits.

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Is not all about memory the other limit is the GPU usage, so you need to try to find what is the best value for your particular system. How to look? You could use the Fred´s program or try to find the value manualy.

To do that manualy, use GPU-Z for example and look the GPU usage, try 2 WU at a time and see, if with the the GPU usage reaches >98% that is your best value, if not try with 3, the best value is the one that give you the GPU usage of >98%.

Using a number of WU greter than that will only make your system less optimized due the overhead of switching the tasks. Thats why sometimes 3 is worst than 2.

A tip, normaly 2 WU at a time put your GPU usage in the >95% range or more, unless you have an extreame fast GPU (heavely OC, etc.)the best value is always 2 or 3, few GPU´s work faster with 4 or more.
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Don't forget the that the AP OpenCL apps have low default parameters so they'll work on low end GPUs,
by putting -unroll, -ffa_block and -ffa_block_fetch parameters into your ap_cmdline.txt file along with the values you want to run, you'll increase GPU utilisation, and memory usage.

Claggy