The answer, as usual, is "depends."
I have a GeForce 256 SDR on a PII 333DMhz CPU, 66Mhz PCI bus (33Mhz AGP bus), so most of my games are processor-limited. In other words, even though I have 192 MB of RAM, because of the limitations of my processor they're only gonna display so many frames per second REGARDLESS of what video card I have installed.
You may (or may not) be experiencing the same problem because of the limitations of your Celeron CPU. That, plus 500Mhz isn't very fast nowadays.
I CAN say that, generally speaking, upping my RAM from 128 to 192 made a SIGNIFICANT difference in the normal operation of Win2K.
As far as games, it didn't seem to make much difference in say, Soldier Of Fortune, where I was more processor-limited than anything else. Not to say SoF was bad; it only slowed down during the last segment when 15-jillion bad guys were trying to kill me, all at the same time.

More RAM may or may not help improve the frame rates of your games. Most modern games make use of the physical specifications of your 3-D card, and are less dependent on CPU RAM than they used to be. In other words, if you have a new hot & heavy 3-D card, they'll make use of it, DEPENDING on your CPU/RAM, etc.
It did make a BIG difference in Unreal when I first got my PII 333Mhz, and upped the RAM from 64 to 128, but at that time I was more graphics-limited than anything else.
That may explain the reasoning behind my, "depends" answer
Basically with games, though, there's more of a chance you'll be processor-limited than anything else if you have a bleeding-edge graphics card. (This also involves your bus speed to some extent.)
For a long, long time it's been thought that computer game demands drive hardware development much faster than software demands do, and I have to agree with that proposition.
When you think about it, my wife's P133 does just as efficient a job in developing software as my PII 333 does, but man, DooM is about the fastest game she can play on it!
Anyway, that's my opinion, and I'm stickin' with it. Good luck!
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"This year will go down in history. For the first time, a civilized nation has full gun registration. Our streets will be safer, our police more efficient, and the world will follow our lead into the future!" -Adolph Hitler, 1935
[This message has been edited by DeadCats (edited 24 September 2000).]