Well u can get the i7 3820 like the alienware has or a 3770k similar priced and performance, but lga 2011 (3820) will be like $250 for a decent board, where lga 1155 (3770k) is about $160, a msi 670gtx $380, Case of ur choice, Western digital black 2tb hdd, $200, $70 blu ray burner lg, $100 win 8, EVGA 750watt gold certified full modular psu $140.
lga 2011 motherboard would suggest a ASUS P9X79 LE, and lga 1155 a ASRock Z77 Extreme4/6
24gb of ram as quad channel or more? lga 2011 has 8 slots for ram where lga 1155 only has 4
Well first of all alienware is not all quality components, they give a crap motherboard thats only slightly better than a reg prebuilt, the ram is cheap non heatsinked with prob the stock cooling fan, so if anything all ur asking for is same cpu and gpu it uses but with better components? U can get an 7i with aftermarket cooling and a 670gtx with nice mobo, psu, hdd, case for like $1300 without OS, and a montior, keyboard, mouse.
I can build one of those, it wouldnt be worth the money to get exact parts as a alienware lol