their not cards first off
their sticks
secondly, the most i've seen ever done on a single stick of DDR is 4GB, it was ECC RAM for servers (how do you think quad Xeons can get 32GB in 1 board? it's like 8 or 16 slots and 2-4GB per slot...and yeah, it costs 5 digits)
but for SDRAM I believe 1GB is the max, but if the system is using SDRAM it'd probably be cheaper to just upgrade (unless it's a cluster, where you have thousands of systems with SDRAM, in which case it's going to be sad compared to DDR, RDRAM and DDR2 equipped systems (it is possible to build a Beowulf that uses RDRAM, it would just cost multiple times what it'd be worth (As the RAM would outweigh the cost of any other single component (unless your talking Petabyte HDD storage ammounts) so it would be pointless, as DDR PC 3200 in single channel can wallop any form of RDRAM (well, i'd like to see RDRAM 1600 Dual Channel (Rambus speaks of it, but i haven't seen it...))