The LGA775 CPU socket served for Celerons, Pentium 4s and Core 2 series processors.
A CPU socket is the connection point for a CPU onto a motherboard; the pins on the bottom of the CPU slide into holes on the socket to make an electrical connection, and different generations of processor sockets (and processors) have different spacing between the pins. The LGA775 CPU socket was used by Intel for a wide range of CPUs released from 2004 to roughly 2008. It has been supplanted by the LGA1156 socket for the core i3, i5 and i7 series processors, but was the processor of choice for two prior generations of CPUs. AMD processors will not fit into an LGA775 CPU socket.
High End LGA775 Compatible Processors
While not all motherboards can support them, the top of the line for the LGA775 series CPU socket was the Core 2 processor series, with the Core 2 Duo E8600, Core 2 Extreme QX9770 and Core 2 Quad Q9650 being the three top performers; all are roughly 18 months off the market as new parts in late 2010 and can usually be had at a discount from vendors looking to clear out old inventory.
Server LGA775 Compatible Processors
Intel marketed the server version of the Pentium 4 as the Xeon 3000 series processor. This processor -- like most of Intel's server hardware -- relied on increased amounts of on-chip cache memory to speed up the processor when used for large data transfers; the focus isn't on higher performance through a faster clock speed, it's on making sure that the gateways carrying data to and from the CPU are wide enough that the chip is never 'idle' while waiting for the next chunk of data. This CPU can still be found on eBay and at some computer part vendors, and has retained a fair bit of its price; the primary market for this CPU are users of legacy servers that need exact replacements, rather than migrating their tasks to different hardware.
Budget LGA775 Compatible Processor
Intel's initial LGA775 CPU offering was the Pentium 4; this was eventually turned into the Pentium D. While they're difficult to find now, the Pentium 4 was effectively die shrunk (where the transistors are reduced in size) and bot sold as the Pentium 4 Extreme, and then (with the chips that couldn't be run at the highest clock speeds) sold as the Celeron. Later in the development cycle, these both became dual core processors; the top of the line for the Pentium 4 was the Pentium Dual-Core E6600 for the high end desktop and the Celeron Dual-Core E3400 for budget desktops. The current market for both of these CPUs is fairly limited and they are difficult to find.
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