I know my 6100 can support 64MB and 128MB 72-pin SIMMs from experience (for a max of 264MB! Woo!). I never bothered to put more than 64MB total in a 6200/5200-series machine because there's really no point to it - it's still going to be dog slow. Those things were engineered to suck. All they do well is various productivity-related tasks (read: word processing/spreadsheets) or small educational programs (like Math Rabbit, The Playroom, Number Munchers). Their motherboard architecture is waaaaay too crippled to try anything serious. In fact, the 52/6200 machines were what gave such a bad first impression of the 603, an otherwise fantastic processor.

If it's possible, the best speed-boost for a 52/6200-series machine would be to swap in a 53/6300-series motherboard, or higher (preferably a 6500/300, at least a 6400/180). You'd want to make 100% sure that a replacement board would fit in the 6200's case before you order one, though.

Otherwise, max'd RAM, Speed Doubler, and System 7.6 is about the best that little 6200 can do speed-wise.