Not exactly true. it really depends on how many apps you are using off the terminal server. my clients use a very memory intensive database app off terminal server. it takes closer to 100 mb of ram for each client.
Again you might want to check your database server & network, because only the first client will use large amount of memory, but that should be considered as the inital operation at start with the OS & any other services that you are running.
Eadh application & the OS will take X amount of memory but it shouldn't be much more than 384 megs of memory as suggested by MS & many other vendors, and the subsequence clients doesn't take much more ram (depending on the apps). Even with Cognos PowerPlay, Impromtu, Excel, Word & email client all open at the same time shouldn't take much more than 40megs per client on MS TS.
I have had DB2, Oracle & DC/Exchange run on various Pentium Pro 200mhz single, dual & quad with 64-128 megs of ram. They aren't the fastest of servers, but they all do able in a pinch.