I hope you realized that this question has been asked a million times and that the search button is your friend. 2000+ posts and im surprised you would need to ask a ? like this...
Anyways, Im a nice guy, so Ill answer you...SLi will work with mixed brands as long as you're getting the same gpu, hence for you, another 7800 gt. I suggest you to get another 7800gt with the same clock speeds as your current one to get your moneys worth because if you get one with higher speeds, it will simply clock itself down to the speed of the lower clocked one. Also, if you OC your card at all, bring it back to stock. Then uninstall drivers---> Driver cleaner it--->Add 2nd card and whatever else is necessary for your mobo to enable SLi--->reinstall driver---> enable SLi...kapisch???
If you need more confirmation that mixed brands will work in SLi...well, my mixed brand SLi setup works like a charm
edit: btw, after you enable SLi, you can then OC again, but more than likely you wont be able to OC as much as you would with a single card