I've seen many others in this forum advise that it's best to wait and buy all components at once.
To some degree I would agree, especially with the main components. I would suggest you NOT buy motherboard, processor (not an issue, since you're waiting for Sandy Bridge anyway), graphics/video card (these fluctuate in price so often, you should wait until you're ready to use it), RAM (same with fluctuating prices), and hard disks (ditto).
I would say "safe" components to purchase ahead of time (in the couple month timeframe) would be: peripherals (kb/mouse/cardreaders/speakers), sound cards, cd/dvd drives (unless you're looking for anything blueray), and display (if you find a good deal). Also, cabling doesn't tend to change all that much (SATA cables, etc), and for that matter, fans and heat sinks.
Hope this helps you out; good luck with your build!
I enjoyed having a projected planned list, and going back every month and adjusting the parts every month based on newegg and amazon and tigerdirect. I was amazed by the difference from November to January in terms of how much more my money got me in just a few short months.
To give you an idea, better processor, cheaper hard drive, cheaper ram, cheaper and better motherboard, more expensive graphics card (upgraded to GTX580).