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The main things that will help framerate is going to be the video card and RAM. With 1Gb and an older ATI, you are going to run into redraw issues since both of these are a bottleneck. The chip is decent so you are fine there for now.

Since you probably only have an AGP slot on the board, you could check out these for examples. I normally use pricewatch but for folks new to PC upgrading, Newegg tends to have some good deals and such.

If you can swing it, I would kick the RAM up to 2Gb and get a 256Mb card.

For video cards:
XFX PVT73AUDF3 GeForce 7600GT 256MB AGP 4X/8X Video Card
Radeon X1950PRO 512MB 256-bit GDDR3 AGP

There are a lot of other cards and opinions will vary but these will give you a starting point.

For RAM, CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800

Now, bear in mind, to make an accurate suggestion, we would need to know what motherboard you have, the type of RAM you have and the other system specs. The links above are suggestions only based on the initial post you made. You can get a program called CPU-z to get this information, available here: CPU-z link