Get the 4870, the 4850 which is single slot will heat your case up very quickly. The Sonata III does not have sufficient exhaust capacity for a high powered single slot GFX card.
It fits even with the optional front fan installed, but very tightly. I have it installed in my set-up. You may need to force the PCI-E power cables just a little bit, but it definitely works. I also installed an accelero with another 120mm fan strapped to it.
The GTX280 and 260 seem to be a little long, so you may not be able to install the front fan.
You are about to answer a thread that has been inactive for more than 6 months. If you still wish to proceed, please ensure that your posting is original and does not duplicate or overlap any prior responses to this thread.