It's not that I'm against on-rails sections. They can be fun. It's that (a) they make terrible finales, and (b) the worst kind of rail shooter is where you're constantly shooting down incoming missiles instead of bad guy space ships, hence why KZ3's finale was so disappointing.
Overall, KZ3's vehicle sections were much better than KZ2's. The tank section was just lame (with 512 MB, you can either have high-res graphical assets or enough space to maneuver), and the turret section was kind of boring, too. But I think KZ2's on-foot sequences were generally more engaging. I didn't particularly like KZ3's jetpack sequences. They were okay, but they were too constrained by invisible walls. Crysis 2 did this sort of thing much better (power jump isn't quite a jetpack, but same idea). I didn't really like the space station battle, either. It was just way too rails-y, and the physics didn't make any sense (why does gravity stop working when someone dies?)
The opening sequence where you escort the convoy was pretty intense. I enjoyed fighting my way through the jungle. The junkyard mission was okay. And the mechs were fun. But most of the infantry combat was either unmemorable, gimmicky, or so linear that it made Call of Duty look like a sandbox game.
Well, kz2 use barely 256MB of ram in the graphic maybe even less from what I remember ...