Don't get the 2Ghz P-M, get the best bang for the buck chip (probly 1.7 or 1.8). But then that won't be much of a performance improvement then, will it? It seems as though leaving it alone is the best choice, yes....
Seriously, clock speed increases don't do as much for performance and $700 on a CPU is dumb. Put 1GB & 7200RPM HDD and call it a day