How much would it cost...

To upgrade this comp, and what would I be upgrading? Here is what I have -
Intel 82915G/GV/915GL Express Chipset family x2
Intel Pentium 4 CPU 3.00 GHZ x2
145 GB SATA NFTS Hard Drive
Dell Dimension Desktop 4600
Plug and Play 16 Inch 1024i monitor.
2 siblings.
Intel 82915G/GV/915GL Express Chipset family x2
Intel Pentium 4 CPU 3.00 GHZ x2
145 GB SATA NFTS Hard Drive
Dell Dimension Desktop 4600
Plug and Play 16 Inch 1024i monitor.
2 siblings.