I want to build a computer right now too, but I decided to wait until fall. If you really want to spend 2k on something then wait till sometime in Sept-Nov time frame since both the 6/12 core Ivy Bridge-E and the new Ivy Bridge E5-2600v2 based 8/16, 10/20 and 12/24 core processors will be coming out. There is also a 15/30 core coming out but it will cost far too much. They are all socket 2011, except maybe the last one. The 8/16 and a couple of the slower 10/20 cores are expected to be less than $1000. The Ivy Bridge-E will run in the $600-$1000 which also means Sandy Bridge-E should continue to plummet between now and then, which also makes it a cheaper option. One good reason for waiting also is we don't yet know if HCC2 will be GPU or CPU. They are expected to announce that later this summer. Then you will know what route to take and know if you need lots of money for GPUs rather than CPUs.

If you don't want to wait then consider buying multiple laptops since right now everything is CPU only. They take little power, produce little heat, have keyboard/touchpad/screen and built in battery backup. I paid only $550 for my 4/8 core Ivy and it makes a great cruncher. You can stick them almost anywhere, but I wouldn't plan to put them in a living room since they can get loud, but so will a cheap pc.

Pandelta,
I just promised my wife no more upgrades and LOOK WHAT YOU POINT OUT!
"Ivy Bridge E5-2600v2 based 8/16, 10/20 and 12/24 core processors"
Now I have to research...

so what did you decide.

Nah, this would be better....
http://www.tomshardware.com/news/Haswell-X-Xeon-EP-Intel,23477.html

2 types of ECC ram, registered and unbuffered dimms. Unless your doing critical nuclear physics work, you don't need registered ECC. Unbuffered dimms are cheaper. Don't buy xeons new unless you get a sweetheart deal. eBay and searches on the web will save you some cash. Xeons are pretty bullet proof but still buy from a reputable place that has a refund policy just in case you do get a DOA. You will want to have mobo, ram and PSU 1st so you can pop them in right away to insure they work. Figure out which ones you want. Suggest Nehelam or better since these have hyperthreading. Or go AMD for less expensive. You can run one processor in a multiprocessor board. Just have to pay attention to installing the ram. That's how I did it. Had 2 duallies built, over time. Sold one earlier this year. One mobo was purchased new, the other was a return newegg sold cheaper. If its just crunching, you don't need all the bells and whistles of a top end mobo. Get a basic one. The dual GPU ones cost more but you will be able to crunch with 2 GPUs if you can afford the electric bill. Pay attention to E and X series processors. There are a little different but the E series are easier on electricity.
For cheap duallies already built, look for harpertowns rigs folks are dumping. Ram for those aren't cheap and run really hot. Strong crunchers though. Do use the electricity though. Had 2 of them awhile back. Actually 4 counting my Mac pros I had.
Not sure if you want a quad processor setup. You will need a big case for that. Bigger than E-ATX from what I checked into awhile back. Could have changed since then. Quad xeons will be expensive and AMD has low ghz. I still consider doing one from time to time.
One thing I have noticed on my dual X5670's is depending on project, write speeds can get bad. I crunch with 23 thread, leaving one free for OS overhead. Also use a SSD for OS and boinc but put boinc data on a hard drive. Some folks crunch on the SSD, I won't yet until they get cheaper.
Tell me your thoughts on what speed you want out of a dually or bigger. The higher the ghz, the more it will cost. I picked the X5670's for good speed and a little easier on the electricity. The bigger ones are 130 watts. Think mine are 95 watts. If speed isn't a priority but core count is, the E5645s are cheap. BTW, what OS are you considering? If you know Linux a little, it's free but GPU support isn't the best but doable. Otherwise get win 7 OEM.