I have the a2630 in mine with 4mb onboard and the speed difference was noticeably huge over the stock machine.
GVP combo boards are plentiful but some only take GVP's own make of memory simms and they can be hard to find and expensive.
Couple an a2630 with a hardisk/memory card like the GVP HC+8 or commodore a2091 and you have a cheap but effective upgrade path.