If you go into the driver settings of the video card and set all the settings to the highest, including the AntiAliasing and Anistropic Filtering and turn on the Aliasing and all the effects in Doom 3 you can apparently lag in some spots even on the Radeon 9800XT. Now this was a while back and both ATI and Nvidia have made driver improvements, with Nvidia cards generally leading in Doom 3. Also note that something like the 6600GT has features and settings not available on the FX5200 to improve visual quality which can impact frame rates when the settings are increased.
As for your freezing, probably caused by too high overclocking or lack of cooling.
Depending on your FX5200 it might be either 64 or 128bit, for the memory interface. The 5700 can be either 64 or 128 bit as well but the 5700LE is no better then a FX5200. Clock speeds aside that will make a big difference - the 9800XT, X800, or 6800 are 256 bit. The 5700's use a different core from the FX5200. Overclocking the memory, GPU, and using the latest drivers while keeping an eye on the temps will work to get the most out of your card but you'd be hard pressed to get it to a 5700Ultra performance level even if you have a 5200Ultra.