The four items I would get right away are:

At least an Intel Pentium 4 processor (or better still a Core 2 Duo running slightly faster than the speed of your current Celeron)

2GB RAM (Running at a minimum speed of 667Mhz)

A second hard disk (SATA - 7,200 rpm) unless you already have a second hard disk. The largest capacity you can afford and put all your video work onto it and nothing else.

A firewire socket if you intend to use Mini DV.

Use the Disk Defragmentation utility much more on the second hard disk to keep it up to speed.