Tartan Giant,
Right now the best bang for the buck is the GeForce4 Ti4200. The GeForce4 Ti4200 will beat the FX5200 and the FX5600.
As for your second question, the card is a direct replacement for your GF2. The only issue you might have would be with your Power Supply. You want to have a power supply that has 28a on the 3.3v RAIL and a Max Combined Wattage of 200W on the 3.3v + 5v RAILs. This information can be found on the sticker on your power supply.
Since you are running WinXP, you might want to bring your RAM up to 512Mb too.
Take Care,
Capt. Richard J. Gentil, Pres.
Naples Air Center, Inc.
Custom Computers of Naples, Inc.