If I'm right, yours would be 2GHz Celeron with a 533FSB on an Intel 845GL chipset, so taking this into account your upgrades are limited to ..
Fastest CPU : P4 SKT 478 2.8GHz 533MHz FSB 1MB Cache Prescott
Maximum Memory : 2x1GB.
Best Video : PCI based ATI 2400 Pro 128-Bit or ATI X1550 or x1300 128-Bit
As always with these SFF's, no guarantees, BIOS may need updating for newer cpu's and power supply may not provide enough power.