Mixing PCI and PCI Express Graphics Cards

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Okay, so I recently created my own new system 100% by myself. I thought I was ready to do so, for having worked on and with computers for several years. However, I'm encountering error after error after error.

The latest one is this; while waiting for my dvdrw drive, computer case, and more ram, I put in my new nvidia 512mb (geforce) 7300 PCI express graphics card to have dual monitors; I only have 2 old crts, and no adapters, so I left the older normal PCI card, a nvidia 6200 series 256mb card, in.

Now, I have completed the assembly of the computer, and tried playing oblivion; to my utter horror, the game looked god-aweful and lagged something worse. In my previous post I listed my system specs; here they are again for reference;

ECS NFORCE6M-A - Motherboard Based on NVIDIA nForce 520LE (Socket AM2) Chipset
4.8 ghz AMD athlon prossessor (Brisbane 2.4 x2 dual core)
2 gigs RAM

I have almost 1tb of Hard drive space, 350gb free, so there's no question of room/space.

I'm thinking it might be a conflict of drivers? It was brought up in the previously mentioned thread that my PSU might be really straining to power eveything in and outside my apevia x-plorer case, but I doubt it because my left monitor (the older crt on the 6200 monitor) always flickers on boot, something it never did until i put both cards in, and defaults to 800 x 600 resolution. I can then change it to 1024 x 768 manually, however.

Any help appreciated =/

I'm running windows XP proffessional SP2 with vista inspirat 1, ftr