What should we look at for a machine?

1. CPU (how many core, how many physical cpu(s), how fast, 64 bits?, cache size)
2. Memory RAM
3. IO speed 

Dual-core CPU vs multiprocessor

A dual-core CPU is a CPU with two separate cores on the same die, each with its own cache. It’s the equivalent of getting two microprocessors in one. Multi-processor has 2 or more CPUs physically on the motherboard.  An attractive value of dual core processor is that it does not require new motherboard. For now, I have heard of quad-core CPU as well (4x). For example, Dell PowerEdge 6950 has 4 quad-core AMD Opteron 8300 processors.In a single-core or traditional processor the CPU is fed strings of instructions it must order, execute, then selectively store in its cache for quick retrieval. When data outside the cache is required, it is retrieved through the system bus from random access memory (RAM) or from storage devices. Accessing these slows down performance to the maximum speed the bus, RAM or storage device will allow, which is far slower than the speed of the CPU. The situation is compounded when multi-tasking. In this case the processor must switch back and forth between two or more sets of data streams and programs. CPU resources are depleted and performance suffers.In a dual core processor each core handles incoming data strings simultaneously to improve efficiency. Now when one is executing the other can be accessing the system bus or executing its own code. Adding to this favorable scenario, both AMD and Intel’s dual-core flagships are 64-bit.To utilize a dual core processor, the operating system must be able to recognize multi-threading and the software must have simultaneous multi-threading technology (SMT) written into its code. SMT enables parallel multi-threading wherein the cores are served multi-threaded instructions in parallel. Without SMT the software will only recognize one core.

Memory is important

If you cache stuff, you need memory. Cache is one of the key weapons to boost your performance. It can reduce the number of database calls dramatically and eliminate unnecessary query processing time. So, you want to buy your system more RAM and it is cheap too!

Disk Storage

When we talk about IO, we are looking at the power of our storage device. Here we are going to understand some of the common terms

1. SCSI - It’s a fast bus that can connect lots of devices to a computer at the same time, including hard drives, scanners, CD-ROM/RW drives, printers and tape drives. Other technologies, like serial-ATA (SATA), have largely replaced it in new systems, but SCSI is still in use.  
2. RAID - SCSI is often used to control a redundant array of independent discs (RAID). Other technologies, like serial-ATA (SATA), can also be used for this purpose. A RAID is a series of hard drives treated as one big drive. Disk arrays stripe data across multiple disks and access them in parallel to achieve high throughput for the system. But large disk array is highly vulnerable to disk failure. The solution to the problem of lower reliability in disk arrays is to improve the availability of the system via redundancy (fault tolerant!). However, redundancy has its disadvantage of lowering the write performance because of maintaining the consistency across 2 replica. Data striping (0) - improve performance; Redundancy via mirroring (1) improves availability. RAID 01 - mirror of strips, RAID 10 - strip of mirror. For 10 machines, you adopt RAID 10. You will have 5 set of mirrors and each set contain its own strips. Clearly, RAID 1+0 is more robust than RAID 0+1.
3. SAS - The newest type of SCSI, called Serial Attached SCSI (SAS), uses SCSI commands but transmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per second.
4. SATA vs SAS. In term of storage (GB/$), SATA is a LOT better than SAS. But in term of performance, SATA is 10K rpm is slower than SAS (15K rpm). In addition, SAS is more reliable.
5. iSCSI - iSCSI is one of two main approaches to storage data transmission over IP networks; the other method, Fibre Channel over IP.
6. SAN - Storage Area Network (SAN) is a high-speed subnetwork of shared storage devices. A storage device is a machine that contains nothing but a disk or disks (disk array) for storing data. A SAN’s architecture works in a way that makes all storage devices available to all servers on a LAN or WAN. As more storage devices are added to a SAN, they too will be accessible from any server in the larger network. In this case, the server merely acts as a pathway between the end user and the stored data.