What is the PCI-express with a ddr2 memory and a nforce 520 model?
CREATE TABLE "series" (
  "model" text,
  "processors_supported" text,
  "fsb_ht_frequency_m_hz" text,
  "memory" text,
  "pci_express" text,
  "sata" text,
  "pata" text,
  "sound" text
);
SELECT "pci_express" FROM "series" WHERE "memory"='ddr2' AND "model"='nforce 520';