How Much Memory Do I Need?
Choosing memory is probably the easiest thing to do. First thing you need to do is decided how much memory you need. Look below
 
Entry System - You might be able to get away with buying 128mb but I would recommend buying 256 as the price difference is very little
Mid Range System - I class my system at home as mid range and I have been using 512mb of memory and I have never had a problem of running out of memory. So I would say buy 512 - 1024mb of memory, If you plan to install and use windows vista I would recommend 2 gig of memory.
Top Of The Range - Buy 1gig or 2gig for a future proof machine.

What Form Of Memory Do I Need?
There are two types of memory, SDRAM which is used in older systems and current budget systems, And DDR (Sometimes called SDRAM DDR) which is faster memory used in most new computers today. You need to select a motherboard before you choose the memory, So check your motherboard manual to see what memory types are supported.

What Is The Speed Of The Memory?
The faster your memory runs, the better your system will perform. With SDRAM it will run at 3 different speeds. 66mhz, 100mhz or 133mhz. DDR memory runs much faster than SDRAM. DDR memory speeds are measured as follows, “DDR PC4000″ the number 4000 repersents the memorys speed of 4,000 mb/s.

What Is DDR2??
DDR2 is the next generation of memory. While the speed ratings of the DDR2 chips might be the same as DDR memory DDR2 runs much faster as it has an increated bandwidth by sending larger chunks of information at a time. At the time of writing DDR3 memory has been announced but little information is available.

How Many Modules Shall I Buy?
For the budget system you will buy one memory module. But for the mid / top of the range system you might want to buy two modules. Most modern motherboards support “Dual Channel Memory” This will mean that two memory sticks will work together to increase bandwidth. This will mean that you will have to buy 2 identical memory chips.