I'm looking at buying a used alienware m14x off Craigslist. The specs are: i7, he says gtx555 3gb, but I don't think alienware offered that gpu, I think he just means gt555, 8gb ram, 1600x900 screen for 600, good deal? Or should I buy a new lenovo y580 for 850? Or should I just build my own desktop? I appreciate any feedback.
Depends on your purpose. If you need it for gaming, I'd suggest building your own rig for about the same price. That way you can be sure that the computer is new. Also, laptops are not generally for gaming. However, the lenovo y580 seems to be a nice alternative if you plan to game on the go. But, if you plan to game in your house or wherever and not take it anywhere, I'd strongly advice you to build a pc.
If you don't need the portability of a laptop, a desktop is the way to go, especially if you have spare parts. If you need a monitor, os, keyboard, speakers, mouse, then it'll be close between desktop and laptop on a low budget.
How much are you willing to spend?
Personally, I'd buy the y580, the 660m is a good bit more powerful than the 555m (3 gb is worthless as you will never go over 1 and maintain playability. If the y580 has the 1080p screen then definitely go for it. I think also there is the y500 on newegg for $899 which includes sli 650m (much much more powerful than the 555m).