user: Question: What is the PCI-express with a DDR2 memory and a nForce 520 model?
Columns: ['Model', 'Processors Supported', 'FSB/HT Frequency (MHz)', 'Memory', 'PCI-Express', 'SATA', 'PATA', 'Sound']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['nForce 500', 'Athlon 64 X2, Athlon 64, Sempron Socket AM2', 'HT 1 GHz', 'DDR2 / DDR', 'x16: 1 slot x1: 3 slots', '4 Ports 1.5 Gbit/s', '4 Ports', "AC'97"]
SQL:

assistant: SELECT "PCI-Express" FROM "Table" WHERE "Memory" = 'DDR2' AND "Model" = 'nForce 520';