For older machines, you won't see amazing strides in performance compared to newer gear. For example, PrimeGrid utilizes AVX extensions. If your chip doesn't have them or are AMD, then they really won't perform all that great "in comparison" to newer gear. This doesn't mean that they aren't worth running as that is all subjective. However, if you put them on WCG, they will perform clock for clock in many scenarios as WCG does not have optimized applications to take advantage of newer technologies. So, in that scenario only power efficiency can really be argued. Since you are willing to run them anyways, that isn't really an argument.
Bottom line, pick a project you like and just run them as long as you are willing. Definitely take advantage of Non-CPU intensive projects as well since they run alongside the CPU intensive work.
GoofyxGrid
WUProp
and if it ever comes alive again Gridcoin Finance
If you have the right detectors, even QCN and/or Radioactive
And don't forget that many older to mid level GPU's can easily be fed on older hardware. If you have the opportunity to add a card to each box later, that might be the best route.