for the full effects of the graphics card, you would need a new motherboard, make sure it is compatible with your current memory and cpu. If not then you'll end up having to totally overhaul your system with a brand new computer.
The thing is, some early PCI-E 1.0 boards were not 100% PCI-E compliant and had issues. Of course, by now most of them have BIOS updates to address that, so you may want to check for BIOS updates for that particular issue. Other than that PCI-E 1.0 only has half the bandwidth of PCI-E 2.0, but that generally does not affect performance that much. With lower or midrange cards, the performance difference is generally nill, and only in higher end cards will it be measurable.