i was working on a pretty tough machine for under a grand, about a month ago, it was coming along real well, but then I read news that AMD was not going to continue dishing out an more socket 754, CPU's, which means this would be the only cpu the mother would ever see. Now they've moved to socket 939 - 940, Which the nicest 939 will start at $800 or so... which is a drag, so I was thinking get the 754, and mother board (combined 300 ish) then when the time came to upgrade use the same ram/vcard and get a new cpu/mother board, we'll see.
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http://www.extremetech.com good info there on building a 1 Grand machine/intel or amd.