I currently have a GTX 260 GPU. I know it sucks for mining, so I was thinking about getting a 5830. If I get one, can I just plug it into another slot on my mobo, and use just that one for mining, and continue using the GTX 260 like I normally do? Would having two different drivers mess things up too much?
Depends on what you do with your GTX 260 and what platform you are on.
Under Linux I have done just that. I use my Nvidia hardware to do protein folding and my ATI hardware to do BTC mining. Getting them both working took a bit of time (the Nvidia hardware works at a different OpenCL revision than the ATI hardware which seemed to cause some problems).
As others have mentioned some motherboards use 16 PCIe lanes to service two PCIe slots. Effectively making them 2x 8x slots when two cards are installed. However 8x is more than enough to do folding so even if this was the case for my setup it wouldn't harm my work.
As a rule of thumb Socket 775 boards that have two or more 16x slots are usually doing something weird. i.e. splitting lanes, multiplexing them, etc... the chipsets for the newer sockets should at least be able to handle 2x PCIe 16x
