find out what mobo you have and google it. you should find specs like CPU Socket Type, DIMM slots, expansion slots. All those goodies.
If it's a P4 1.8ghz, it's probably Socket 478. May as well get a new mobo and CPU.
Is your video card PCI x16? I would assume not... but if it's PCI, then maybe you could grab the bundle I got off Newegg. $137 for a AMD 3400+, Asus A8R-MX mobo, and Freetech (Actually a Winfast in a Leadtek box) Geforce 6200 PCI x16. I have it unlocked to 8 pipes and it's at 600mhz core and 671 memory on stock cooling. At max, my Core is at 100F and my CPU temp is at 85F.
My friend's Celeron D 2.8ghz idles at 115F. I think he needs new thermal. AS5!