My opinion is that you would find it hard to get any decent graphical performance on the games you stated with the GeForce FX5500. I mean your original question was what is a good core clock speed, and well the highest end cards (specifically the ATI Radeon X850XT) have about 540MHz, and another example (specifically the nVidia GeForce 7800GTX) will have from 400MHz up to 490MHz. So the 290MHz from the FX5500 is really brought into focus when you compare it to the top end cards.
But rememeber that you can overclock your graphics card to get more out of it. Some people have overclocked the nVidia GeForce 7800GTX (mentioned above) to 720MHz core clock speed (nearly double!). But that is an extreme case, and if you were to over clock that FX5500, you could get around the 350MHz figure I think.
Now unfortunately, as you're looking at cards like the FX5500, I guess you have an PCI motherboard which limits the graphics cards you can get. This means that the ATI Radeon X850XT and nVidia GeForce 7800GTX is out of your league unfortunately
I'm not too sure on how the PCI market is at the moment, but that card you have there I think is one of the best PCI cards out. Unfortunately, Osak thinks you have an AGP motherboard, but you don't and so you cannot get a 6600GT to fit your system, but as he said, try Newegg for those cards, and try to avoid Bestbuy.
Hope that helps!