The x4 doesn't matter with such a low-end card. It definitely sounds like something's wrong with your setup because of the similar setups' performance.
PCI-e 1.0 x4 has less bandwidth than AGP 8x.
The 5670 performs closely to the 9800GT, so it's not really all that low end.
The PCI-e x4 lanes come from your Southbridge, not your Northbridge (like the x16 lanes), so that may be a problem.
Southbridge lanes are not ideal for graphics cards.
Edited by sLowEnd - 10/9/11 at 2:09am