Here's a good test with different cases (4/8 GB ram, HDD/SSD):
http://macperformanceguide.com/Optimizing-MacBookProCorei7-Combined.html
That's just Photoshop though, but it gives a good idea how the options work. Now already the "diglloydSmall" test is a quite ram-hungry one, exceeding 8 GB. But as the swapping, after the ram runs out, happens on a really good SSD instead of an HDD, the 4GB/SSD combination is almost as good here as 8GB/HDD. Even more so with more load. The SSD is "not quite ram", but makes a fast virtual memory (and dozens of GB of it).
Therefore, adding the everyday snappiness, application launching, zero noise, etc. of SSD's, it seems to me like a better upgrade than 8 GB ram. The ram and a decent size (120 GB) top of the line SSD also cost about the same (here in Europe I think the ram could be found for less in the US though).
So to your original question: if you can afford both, yes it helps some. But after the SSD, not that much

Bottom line: much worse performance per dollar upgrade now than the money you already put into the SSD.