The iMac you're talking about is a revision C iMac I presume, the only 266Mhz model availible. Your iMac has a RagePro card with 6Mbs of VRAM. Should you find only 2Mb in place it was taken from its original place, they were all delivered with 6Mb, only revision A came with 2 Mb.
If of course there's only 2Mb present you either
a) have a revision a iMac
b) need to buy an extra 4Mb if you want some decent quality, you can go to your local hardware dealer and ask for SGRAM, if you want I can look up the reference code in the store tomorrow ...
Hope this helps,