My mobo is an Asus P6T Deluxe V2. I'm sure it can run 2 cards at x16. I'm on Windows 7. I plan to continue using the GTX 260 for normal stuff. I was going to use the HD5830 as a dedicated miner 24/7 because I leave my computer on all the time anyway. So, would I be better off selling the GTX 260, and getting a couple of 5830s? Or even a 5830 and something less powerful, seeing as I don't game much anymore?
That's an x58 board so it can have up to 36 PCIe lanes. So you're right both slots should be able to run 2x 16x no problem. The third slot is probably shared or downgrades the second slot but that doesn't matter. Those slots are so close together that you wouldn't be able to use both of them anyway.
The ATI cards are generally as good as the Nvidia cards at gaming and 2-3x better at mining. So swapping the 260 for even a mid-range ATI card is probably a good idea. You can see the details here:
https://en.bitcoin.it/wiki/Mining_hardware_comparisonRight now the only thing NVidia is better at is folding and that may change with the next rev of the GPU client.