You generally can't do that. Specifically for 10.4, you can disable the Dashboard which otherwise eats quite a bit of RAM, but that is pretty much it. Tricks exist for disabling Spotlight, but if you do that, I don't see why you upgraded in the first place. Spotlight and Dashboard are the two big reasons to run 10.4.
I'm running 10.4 on a slotloading iMac G3/400MHz, although with 512 MB of RAM. This machine has had everything from 9.0 up to and including 10.4.11. After switching to OS X, it got faster with every upgrade until the switch from 10.3 to 10.4 - that was a step down, because it was running out of RAM. It wasn't too bad, and I've kept it on 10.4, but it was a definite step down.
If I were you, I'd stay on 10.3 unless there was some feature in 10.4 I really needed. If I really had to move to 10.4, I'd upgrade the RAM first.