If i was to use boinc on either a 8400GS or a I5 750 at 3,8Ghz (using only 50%), which would be better at crunching data?

I know that GPUs crunch much faster than CPUs but since it is a really old and weak GPU, would it still crunch much faster than a CPU while using less power?

Thanks in advance

I'm not sure if i'm doing this right but seems that my i5 750 is able to do 3,8Gflops according to this:
http://setiathome.berkeley.edu/show_host_detail.php?hostid=7272442

While a Geforce 8400GS PCI-E does 5Gflops according to this :
http://www.gpugrid.net/forum_thread.php?id=1150

What i don't understand is if that CPU rating is the maximum that my CPU can do at 100% or what it can do while at 50% (it's current state at boinc)

If the rating for the cpu is at 100% then i'm only crunshing at 1,9 Gflops on my cpu (3,8/2=1,9Gflops because i'm only using half of my cpu for boinc) while with a Geforce 8400gs it would do 5Gflops. The deciding factor would be power consumption. While my CPU power consunption increases 30 to 45 watts while crunshing at 50%, i'm not sure how much power does a 8400gs consume. I've seen people saying it won't go above 25 watts while others sai it will do 71 watts.

Not really sure about any of this.
Any help is appreciated.

Neither matters for BOINC - BOINC itself uses the minimum resources possible.

Which is better for the science project(s) you have chosen will depend on the project programmers. It's better to choose which area of science you're interested in first, and then read the forums belonging to projects active in that kind of science. Most will have efficiency discussions already, and most will also have a "Question and Answer" area where you can post before you've done any work for the project. Once you have done some work you can ask the project specialists directly.

It isn't all about speed. I doubt a 8400GS with Compute Capability 1.1 will be able even to run the current applications at GPUGrid.