The 512MB 1950Pro & 7950GT (1.2GHz version) are in the $220-240 range.
The higher cost of the latter may be due to its recent release but it does benefit by being faster, having lower power demands and smaller size. However, some performance may be the result of lower quality defaults on Nvidia's part and they are not capable of HDR and AA simultaneously.
Either way, you might consider overclocking the CPU to eek out more oomph. P4's are dead cheap though so you could even buy a faster one (or a slower one for ultimately higher performance after overclocking if the rest of the system is capable) and sell the current one to recoup much of the cost. Heck, if games are the only consideration (as opposed to multi-tasking) then an even cheaper Celeron D could be "re-clocked" to beat the P4 without actually overclocking the system (NB or RAM).