You can always replace your motherboard and keep the case.
However, you must decide what your new system will be. If you choose to go with a Sandy Bridge CPU ("second-generation Core"), the motherboard must support LGA 1155 and one of the compatible chipsets, the choice of which depends on what you plan to do with your system. Your case also puts a restriction on the form factor of the motherboard: ATX, micro ATX, or mini ITX.I suppose the Antec Sonata can accomodate any of the form factors, but it's always safe to know for sure.
Next consideration is RAM. Your present RAM sticks are likely incompatible with the new motherboard. 240-pin SDRAM DDR3-1333 is typically what you need these days.
Next are considerations for SATA 6Gb/s and USB 3.0. Likely your present Sonata case does not support front-panel USB 3.0 ports, but that may not be a problem for you because most LGA 1155 motherboards do include USB 3.0 ports in the back. However, you should take advantage of SATA 6Gb/s, at least for your boot/system/apps drive. In general, you should re-consider your present drives.
Finally, do you plan to take advantage of DVI or HDMI for your display? Most current Intel motherboards do not support VGA (D-SUB) port, serial port, parallel port and floppy drive. I personally like this direction, but it may be a problem for certain PC builders. If you want to go with DVI, say, you may need to upgrade your display.
Oh and don't forget the PSU (power supply). Do you remember the wattage of your present PSU? If it is around 300W, you may be able to re-use it with a new CPU but likely without a discrete graphics card. All Intel Sandy Bridge motherboards have specifications in PDF that you can download to read to find out about power requirements.
Where should you shop for a new motherboard? If you live in the US, I recommend newegg.com. Their website is very well organized. Beside product selections, descriptions and user-reviews, there are some pretty good tutorial videos.
Now, to be more concrete, I suggest you start by looking at the new Intel Core i3-2100 (with integrated HD 2000 graphics). Then select one of the Intel LGA 1155 motherboards that suits the form factor for your Sonata case. (If you still want to use VGA, take a look at the other brands, such as Asus, Gigabyte and MSI.) Then select a pair of RAM sticks, and I suggest 2 x 2GB for a total of 4GB. Then select a new SATA 6Gb/s drive for your new boot drive.
That's about it. Good luck and have fun!