According to the NetBSD 5.1 installation procedures, you'll need at least 4MB of RAM in your TT030 to run NetBSD Atari. If you have less than 6MB RAM you'll need to follow some special procedures described in the document. There is some additional information in the NetBSD / atari FAQ. It's unclear if the 4MB / 6MB requirements applies to ST RAM, TT RAM, or both.
My system has 10MB of ST RAM and 16MB of TT RAM. The previously mentioned FAQ reports that NetBSD only uses the slower ST RAM on the TT030 for video and floppy. The faster TT RAM is the main system memory while you're running NetBSD. The Atari TT Homepage reports that the TT030's memory can be expanded up to 256MB with the appropriate RAM expansion board, but these appear hard to come by.
I have the Atari standard 16MB RAM board in my system.
Although the standard Atari memory expansion board supports a "burst mode," it's not something you can use with any normal SIMM memory. (I tried 60ns 4MB FPM 30 pin SIMMs (from here) just to find out and unfortunately they didn't work in burst mode. Later I did a little reading here in the TT RAM Upgrades archive and found that in order to use burst mode you need a special (fictional?) "nybble-mode" RAM. If anyone is able to find a source for nybble-mode RAM, please update this article. Supposedly the performance impact that you'd get with burst mode isn't anything spectacular though, in any event.