well actually this depends on what you really got under the hood.
There are some rules that apply while selecting a project (grcpool has an own page for that somewhere)
- never crunch a project with a CPU that you also can crunch with a GPU (GPUs are faster will have more RAC and you are competing with them in the end )
- look for projects that have a low overall RAC or low number of users so you wont compete with the whole world :)
you can see a lot of this information and even a lot more on Gridcoinstats - if you sort the project page there by RAC ascending you will get Cosmology@home at the top and LHC at 3rd place which make both a valid pick
what you also can see on this page is the max RAC for different kinds of CPUS, this is on the site for each project, so you can determine what your CPU may be up to in the end.
what i also use as an info is the whitelisted projects where you can see all the projects with all the possible crunchers and you can identify easily which ones NOT to do with a CPU :)
I will definitely check this out. I read Amicable Numbers is a good one to assign a GPU to but for some reason I'm not getting any jobs from them... Thanks for the tips!