WRT54G V2-V4 were fairly consistent - board spins perhaps, but generally the same SoC/WiFi/RAM/Flash, the GL was a re-intro of the V4... or perhaps V3.1 - I've got a couple of WRT54G v3's sitting on the shelf as "keepers"...
V1 was always *WRT friendly, but they weren't that common compared to the other versions, Linksys moved over to V2 pretty quick, as it had a much lower BOM cost, and upside was that V2 had a faster Broadcom SoC... so win-win...
Recall the WRT-54GS models - they were nice for a bit... bit more flash, bit more memory, some challenges with the wifi drivers, but most worked it thru... most of the GS models were linux based, at least the ones I played with, but I've heard that there was at least one version that was VxWorks based there as well... my experience with them was thru V3, which was 32/8 and linux friendly...
WRT54Gv5 was horrible, V6 thru V8 (same form factor) - all VxWorks and making the most of the WRT54G brand perhaps... and then we had the UFO model at the cusp of 11n...
Savvy folks had to really know how to read the codes on the AP retail boxes, as Linksys put the SN's there...
On the client side - total shotgun - same model number, but totally different chipsets from version to version... some were linux compat, some were not, and even within Windows space, some versions were better than others...