Upgrading your RAM to 1GB will help in most new games like Far Cry, UT2004 and Doom3. Half Life 2 will probably also benefit from it.
I have PC3200 RAM in my AthlonXP 2800+ computer and it runs at PC2700 speeds. Unless you want to overclock your processor, PC3200 RAM probably won't get you much, although it may have better CAS latency.
If you have 3 RAM slots, and I think pretty well all nForce2 motherboards do, you could just buy a 512MB stick and plug it in. You will need to read the motherboard manual though to see what slot to put it in to enable dual channel memory though.
I also have a Radeon 9800 Pro and it does well in Far Cry and UT2004, but is starting to show it's age in Doom3. While not bad, it definitely doesn't compare to the new GeForce or Radeon cards. Although for Doom3 my processor is probably holding the computer back just as much as the video card.
One word of caution, many people upgraded last year for Half Life 2, and got burned when it didn't show up. You may want to wait until it comes out see what people say about it's hardware requirements first.
goates