Well, I'm assuming you're referring to the X68/LGA 2011 SB CPUs as the ones which you'd want to upgrade to later.
If you bought a new system now, you'd have to upgrade the CPU and motherboard regardless, seeing as the current Sandy Bridge CPUs, the most high end being the i7 2600K, use P67 (and others) chipset based LGA 1155 motherboards, while the old Core i7 enthusiast platform X58 can only use LGA 1366 CPUs.
Knowing Intel, a new LGA 2011 CPU and X68 motherboard will be quite expensive, although relative to most high end systems, it would be reasonable I suppose.
Also, you'd probably 'need' to add more RAM to make it quad channel, need being in inverted commas as I doubt it'd actually make much of a difference but it'd max out the system's capabilities nonetheless.
Is my current system slow? Not exactly. I have a Nehalem 8 core (16 thread) MacPro with 32 GIGS of RAM but a middlin' GPU. My Mac is screaming fast at most things, save some of my 3d applications, where though it excels at render performance it's very mediocre with OpenGL.
3D is my livelihood so I'm considering using Mac and Windows in mixed network. (I already run Windows 8 sometimes on my Mac).
Apple is perpetually limited in their support of GPUs, which, of course, is becoming all-important these days.
I want to run 3 or 4 monitors and still have lots of VRAM and GPU oomph leftover. I also want CUDA (both for upcoming rendering engines and for Premiere's Mercury Engine), but Apple isn't playing nice with Nvidia and driver support is horrible for any and all Nvidia GPUs on the Jobsian platform. None of the top AMD GPUs are available for the Mac.
I have a MacBook Pro with the new Sandy Bridge and know what a quantum improvement this processor can deliver. I am intent on waiting for MP workstation caliber SB chips/motherboards before building a PC...and when I do I can finally enjoy the best GPUs on the market married to a superior CPU.
I plan to set up an L shaped desk with Mac on one side and a Windows monster on the other.
I didn't mean to belittle whatever system you had before. Either way, I'm sorry if that's how it came to you.
On the topic at hand, if you're going to build around an LGA1155 system, then you'll only need to upgrade the processor in the future. Otherwise, you'll have to upgrade both the motherboard and CPU, as has been said already.