FPS depends upon lots of factors . CPU , GPU , Harddisk Speed , Ram ...Motherboard used ..
Ok your present Configuration is p 3 700 Mhz so what one can guess is this .....
1) You have a PPGA socket .
2) You have 810 or 815 Based Board.
3) You are using SD Ram
4) AGP 4X ( Not that it will matter much here )
Now all this gives you 2FPS in Pain Killer ( Wonder how you got this low FPS and were able to actually count it )
What P4 4 Ghz ( 4 Ghz you said ....hmm now down to eath take that 3 or 2.8 ghz ) will do for you is here :
1) You will have P 4 with Higher FSB
2) An 865 MOBO With higher FSB
3) You will have a DDR Ram..( May be 256 MB )
4) AGP 8X ( Again 4X and 8X is not a difference worth nothing )
Considering the GPU or Graphic card is same You will have hmm far as i can imagine and guess ( Guess because there is no known benchmark to gauge that )
You will have 28 to 35 FPS depending on your setting .
What this means in a nutshell is 25 FPS is playable ..and good for human eye. So yes an upgrade will help you
Luck