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A video card upgrade would provide the greatest benefit. 512MB of memory is more than enough to play any current or near-future game on the market.
However, your GeForce-4 will slow to a crawl under serious shader loading, and most new games will rely upon programmable shaders for advanced rendering.
A 128MB video card will suffice, and the Radeon 9800 Pro is definitely a great choice. However, prices are likely to soon drop due to the near release of the NV40 and R420 chipsets, so you might be best served to wait on purchasing a video card until the games you are interested actually appear on retail shelves.
Robert Richmond