I would be a little bit careful and check several stuffs first.
DDR1 platform was in the middle of transfer from SDRAM ro DDR.
In this era, if I can still remember correctly, the GPUs started to move on from AGP 8x to PCIe x16.
Man..that is quite a long time ago.
Yes, DDR3 on GPU has noting to do with DDR1 on mobo, yes, this is correct and already clarified by those people above.
The only thing you must check first is, does your mobo has PCIe x16 for the GPU or has still the AGP 8x?