Ned is right.
Although the Rambus RAM in your computer was very high end in its time, it is seldom used today.
You really need to buy a MB/CPU/RAM/Video card. It might be tempting to get an AGP style motherboard and reuse your 4600. But just bite the bullet and go PCI Express. If you shop around, you can buy the 4 components for a very reasonable price. (Re-use your all your other stuff till you can afford to upgrade them).
One of the advertisers on THG is Newegg. Great place to get parts. (If your willing to risk it you can buy factory refurb parts for dirt cheap from them).