With a 2k budget you can build a great computer. Are you going to reuse your old screen, mouse, keyboard, case, etc??
I would go with a x79 motherboard but theres only like 2 cpu's that support that socket and cost a lot. So I guess you should use the z68 with a nice i7 2600k. Easily overclockable to 4ghz. Add in a quad channel 16gb ram, buy a 64gb crucial m4 SSD for boot up and another hard drive or two or whatever your taste is. For graphics card, well you can go with the best ATI that just came out, 7970 which is going for 550 on newegg. Or if you want to wait for nVidia's upcoming card then just get a 560 gtx TI and use the step up program from evga to get their newest card (You just pay the difference)
Theres a lot of options out there for creating a rig for 2k. it just depends what you have to work with and what you need/want from the rig