What memory do I buy to upgrade my base Apple iigs memory expansion?

standard IIGS does not use DIMMs. Indeed, I can't hink of ANY II line that use DIMMs.
You want 41256 (256Kbx1) or variant thereof DIP DRAM chips. Most of the time I salvage them off old PC ISA RAM cards

I got a bunch from old IBM AT Clones..

If your willing to go for new Hardware, I have been thinking of getting one of these, 4Meg GS RAM Card. Combined this with a CFFA 3000 Card, and you will be set!!

It´s useless guessing around and proposing advice, without further knowledge about the model of computer....
this leeds to mistakes...

you should first issue more information due to the different versions sold...:
which version is your IIGS ( this is displayed at startup at the bottom of the initial screen ) ?
ROM 0, ROM 1 or ROM 3 ?

it´s also usefull to publish here the typ from serial label at bottom of IIGS.
best option also placing link here to "detail picture" of Mainboard at RAM area.

There are a bunch of different Memory expansion cards available.
for basic info ( pictures and names ) take a view at:
http://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Interface%20Cards/Apple%20IIGS/

And answer about DIMM is only partially correct...
there is a card usable with 4 x 1 MB SIMM Modules ( Garber ) from Briel....:
http://www.brielcomputers.com/wordpress/?cat=22

speedyG

And answer about DIMM is only partially correct...
there is a card usable with 4 x 1 MB SIMM Modules ( Garber ) from Briel....:
http://www.brielcomputers.com/wordpress/?cat=22

Hello dorkbert,

correct! of course DIMM is not same as SIMM....

but a lot of newbies and poeple without electronic knowledge
abuse the term DIMM just for RAM Modules....

and reading my reply you see, that i use the correct term: SIMM ....

and this new member ( just joined a day ago ) seems to belong to that category.....
otherwise he would have noted the question better including the
mentioned missing info´s.... he does not even seem to know about
the different versions of the IIGS... and the fact that this affects which kind
of probably empty chipsockets might be on the mainboard....
nor does he know that some models have been using 4bit wide RAMchips
and others using 1 bit wide RAMchips....

so as conclusion from the content of his question:
it seems he has once seen a Briel RAMupgrade, assuming erratic that to be DIMs...
speedyG

Go to Jameco and buy all you need. Done.
You are welcome - Javster

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?search_type=jamecoall&catalogId=10001&freeText=41256&langId=-1&productId=41398&storeId=10001&ddkey=http:StoreCatalogDrillDownView

again, you want 41256's. These are equivalent. Don't pay more than 25 bucks, including shipping.
disclaimer - not affiliated with seller (etc etc blah balh balh)

I'm going to suggest that if you upgrade that card to 1mb that that if you ever get a hard drive, you'll likely end up tight for memory. Most people consider 1mb to be a little skimpy. 2mb isn't bad. I'd suggest looking for a larger card. There're new and used ones available