Many people who are new to hardware buying stuff usually ignore good Ram for their PC they just think its a 1GB or a 512MB means its good they dont look at the specifications of the one they are gona buy (who know you just land up geting a good ram at low price)
AMOUNT :
The more memory you have, the better your PC will handle running several applications at the same time. For Windows XP and general office tasks, you should have a minimum of 512MB. Multitaskers and gamers should opt for at least 1GB, as should anyone planning to upgrade to Microsoft’s upcoming Windows Vista operating system. If your motherboard supports dual-channel RAM, buy modules in identical pairs to optimize performance.
TYPE AND SPEED :
The maximum speed of memory your PC can handle is determined by your motherboard’s chipset. Vendors such as Corsair,OCZ, and Kingston offer system-scan features on their Web sites that automatically determine which RAM is best for your PC. The most common memory type today is DDR SDRAM, with speeds measured in megahertz (the higher the better). DDR is backward-compatibleâ€â€you can buy faster RAM than you need, but doing so will result in improved performance only if your components support the higher speed. DDR2, the second-generation DDR standard, enables higher clock speeds and data rates. DDR2 won’t work in systems that support only original DDR.
LATENCY RAM :
stores data in rows and columns. Column address strobe (CAS) latency (sometimes listed as “CL”) is the number of clock cycles it takes before a column can be addressed on a RAM chip. In most cases, a lower number indicates better performance
ERROR CORRECTION CODE (ECC) :
Some RAM uses ECC to detect and correct single-bit errors. This is only necessary, however, where data integrity is of the utmost importance, such as in workstation PCs and servers. If you’re shopping for a home or small-business PC, it likely supports non-ECC modules. Buy only the type that your motherboard supports. (Some boards support both ECC and non-ECC modules.)
VALUE VERSUS PERFORMANCE : (i’de suggest ppl to pay more attention to this section)
Memory vendors often offer both lower-cost “value” modules and pricier “performance” options. Only users who need speed (gamers, digital-video editors) should pay extra for performance RAM.
Here is a link for ram and its latency optimization
RAM and Latency - What you Need to Know
Here is an image i found out few days ago on some site forget the price since its in $ so it helps as a decent guide to know each ram specification
http://img146.imageshack.us/img146/5132/ramdetxr8.jpg
http://img408.imageshack.us/img408/7316/ramdet2cp9.jpg
I hope this help someone in some way 
