The best CPU for the 531s is a 6000+ 3.1Ghz Brisbane. Just be sure to update the BIOS to the latest version before putting in the new CPU. As for a quad the answer is no. The motherboard will not support a quad.
Vostro 460 – Intel Core i7 2600 @ 3.4Ghz - 16GB DDR3 @ 1333MHz – Western Digital 500GB SATA 3 SSD – Western Digital 1TB SATA 3 HDD – ASUS GeForce GTX 550 Ti 1GB DDR5 – 16X DVD-RW – 16X DVD-ROM - Antec 550W PSU - Windows 10 Pro x64 - Dell E2211H Flat Panel Monitor