T5500 w/500GB drive $160 http://www.ebay.com/itm/171985608656
2nd CPU/memory riser $80 http://www.ebay.com/itm/331702355304
X5670 x2 $200 http://www.ebay.com/itm/311479761117
24Gb RAM $60 http://www.ebay.com/itm/161176438347
Upgraded heatsink and fan $20

$520

Those are all buy it now prices, but if you're patient, you can snag some bargains and build it cheaper.