First off, I'd re-install your Forceware drivers (also make sure you update your drivers in device manager). Assuming, that is, your current drivers support your PCI '5700. Your mis-matched DIMM's won't be causing trouble unless they were already (although I doubt they're running in dual channel...)..
Anyway, Try that first, and then Forceware will tell you what your card is running on (PCI, AGP etc).
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