I run it on a G3-900 w/640MB RAM and a 32MB ATI 7500, was pleasantly surprised with how well it runs.
There's no official MSR for Enemy Territory (mac version), but the original RCF ones are probably similar. That probably explains your issue. I have heard that it's not suggested to use any Jaguar version for Enemy Territory...Panther only.
The MSR for the original Return to Castle Wolfenstein (Mac version) are:
G3/500MHz or faster, Mac OS X v10.2.8 or later, 256MB RAM, ATI Radeon 7500 (or Nvidia GeForce2 MX) or better 3D graphics with at least 32MB VRAM.
MSR for PC Return to Castle Wolefstein: PII 400, 128MB RAM, 800MB HD space, 4x CD-ROM, 16MB 3-D video card.
MSR for Enemy Territory: PII 600, 128MB RAM, 800MB HD space, 32MB 3-D video card, broadband connection (56K possible for less than 8 players, but banned by most servers).