Is this yours? -> HP - Compaq dc7800 Series Convertible Minitower Desktop/PC
Memory:
DDR2 PC2-8500, DDR2 PC2-6400, DDR2 PC2-5300
Memory Type: DDR2 PC2-8500, DDR2 PC2-6400, DDR2 PC2-5300, DDR2 (non-ECC)
Maximum Memory: 8GB
Slots: 4
In most cases, when u have DDR2, u cannot use DDR3, as it is different from DDR2.
Unless there is a slot for DDR3 too.