Without knowing your specific motherboard model number, it's hard to be 100% sure, but based on the
Product Comparison Chart on NVIDIA's
nForce 500 Series for AMD page, the 590 SLI is only compatible with AM2 processors, not AM2+, AM3 or AM3+. And according to AMD's
Desktop Processor Solutions page, there are no quad core AM2 processors.
So it looks like you're a bit out of luck with your current mobo. The good news is that you may be able to find a transition board that will let you upgrade one piece at a time. Again, it depends on the exact CPU type, and also the exact RAM type, that you are currently using. With AMD processors you have an easier time than with Intel processors since AMD has used the same physical CPU socket for a long time. Since you have a GTX 260, we know that you're already on PCIe, so you also don't have to worry about finding a board with an AGP slot. So it all comes down to whether your RAM is DDR2 or DD3. There are some boards that have slots for both DDR2 and DDR3 RAM, so you could keep everything the same for now (assuming you don't have four sticks of DDR2) but change your motherboard and upgrade your CPU. I did this a while back going from an old Socket 478 P4 (using a 478 to 775 socket adapter), AGP video card, and DDR RAM to a Q6600, PCIe GTS 250, and DDR2 RAM.