Probably depends on what you'd be using it for. If you do graphic intensive stuff where video card makes a difference, then you need to make sure you get a decent vid card too. Desktops at Costco are pretty nice deals but come with really outdated video cards for some reason. Specifically, this one seems like the best deal there:
http://www.costco.com/Browse/Product.aspx?Prodid=11543945&whse=BC&Ne=4000000&eC at=BC|84|56671|78519&N=4047238&Mo=10&pos=1&No=1&Nr =P_CatalogName:BC&cat=78519&Ns=P_Price|1||P_SignDe sc1&lang=en-US&Sp=C&ec=BC-EC10600-Cat56671&topnav= Quad-core AMD w/ 8 GB Ram 1 TB HD for $600 and comes with Win7.
Now if you want high quality stuff, i.e. paying somewhat of a premium but for parts that'll last, cyberpowerpc.com seems to have the best prices on such stuff. You know sites targeting gamers will have good shit since gamers will spend sometimes $5k+ on a dream PC. When I get a new desktop, I'll be getting it there and ultimately probably spending about $1,200-$1,500 on it.