The older 2006 models with a Core Duo can only take 2 GB, late 2006 and early/mid-2007 models can go up to 3 GB (you can actually put in 4 GB, but the system will only see 3), and late 2007 and 2008 BlackBooks can go up to 6 GB.
Since the pre-unibody MacBooks only go up to SATA II in terms of drive speed (pre-2009 machines just use SATA I), any SSD of any capacity will work just fine.
RAM is undoubtedly the bottleneck. I got a 2006 Black, even with a rather slow SATA connection, but as long as the SSD you put in isn't terribly small in capacity, it should fly. But the amount of the RAM is still an issue, even installing 10.6.8 which is IMO the best version for this gen machine, and as undemanding this OS X is, you will find modern daily tasks like browsing already chokes on a regular basis.
I think it really depends what purpose you intend the Macbook to serve. Mine is just a novelty machine looking cool in the studio, with occasional iTunes media playback or ad hoc browsing / photo slideshow etc. But trying to do slightly intensive tasks on it can be painful.