GVP simms are hard to come by... 4Mb'ers are out there in small numbers. 16MB'ers are very hard to find and when (if) they ever surface thery are expencive as hell... often surpassing what you have to pay for a 64 or 128Mb EDO simm.
Anyway... the GVP board should come with some memory, see if it's enough for your needs and if it is then buy... don't think too much about maximum amount or how hard it'll bee to find that extra bit of memory that you'll never get to use. 8-16Mb is usually enough for most day to day usage.