beige g3 needs its OS on its own partition within the 8GB limit. you can use larger drives; you just have to partition the first drive as 8GB and install the OS on it.
you will still be limited to 120GB total, unless you use the 3rd party driver for large drive support over 128GB.
TRIM is not that big of an issue but if you're really concerned with it, buy a drive with over-provisioning which is how the problem was handled before the days of TRIM. Look for drives slightly smaller in capacity to the "normal" sizes, i.e. 120GB vs 128GB, 60GB vs 64GB, etc... those are over-provisioned drives. Basically what over-provisioning is, it allocates a section of the drive as "not usable" and shuffles data around on the drive transparently to prevent a certain cell from being written to more often than other cells on the drive, which will reduce its lifespan.
its also important to note, that this is *ONLY* applicable when you are using the internal IDE bus. it does not apply if you are using an add-in IDE/SATA card, since those drives present themselves as SCSI devices which are not subject to the drive size limitation issues.