Dimension 4300 cpu and ram upgrade.

I just got my sister and brther in laws computer working. They are kind of down on there luck so I was thinking of upgrading some of the guts as a gift. The computer runs pretty slow by my standards (1.5ghz p4 cpu, 512 ram [2x256].  The bios is A06.  I have researched enough to find out that the ram can be upgraded to 1gb. Newegg has a pretty good deal on some stuff there. My questions concern a cpu upgrade. People's success with this seem to be hit or miss. It seems the deciding factor is the mobo model # or the chipset. I want to do this quickly and on the first try so I can send it back to them. Does anyone know what info I need to get before I decide. I would really like to install a SL7EY (2.8 ghz p4). Any help would be appreciated. Would I have to upgrade the heatsink or fan if I did this?