download a program called sandra 2004, tells you everything about his comp..
I would presonally recommend you get the ddr400 ram (pc-3200) but im not sure if that woujld work well with the slower ram chips.. for example the 2100's and such..
_P4 Northwood 2.8C Ghz HT
_Kingston 512 DDR400 Ram [2x 256]
_Awesome looking white plain case with cool green and red power/HD lights, and a newely acquired Intel P4 sticker!