I suggest you get a 1.6A over the older 1.5 and 1.6 Williamette's(all are S478). The 1.6A has 512KB of L2 cache vs. 256KB on the older core, which really makes a difference in CPU intensive tasks. It is also cooler running due to being .13 micron. Cost is the same as the older core(~$135 for a retail chip).
The stock heatsink for S478 P4's is really really good for a retail unit. I'm currently running it and have my 1.6A overclocked to 2.4Ghz and hit 50C full load. It hits about 43C at stock speed, so it is definitely more than adequate.
Hope you like your new rig, I know I definitely like my P4 rig(never thought I'd be saying that either😉).
Defster