memory

[regan1]

I have a Dell C521 pc , I want to up grade the memory.
generic is about half the price of the branded but is it as good
performance wise ?

[chroma]

Branded is just that, Generic with a badge.

Memory is memory, all thats important are the numbers:

DDR2 667 PC5400 3-4-4-8 for instance

WTF do all these numbers mean?
DDRx
DDR memory (dual data rate) comes in 3 different versions (currently) DDR, DDR2 and newer DDR3.
These all run at different speed ranges DDR being the slowest to DDR3 being the fastest, different motherboards and processors support different types DDR2 currently being the most common from mid range systems 3 for newer high end and plain old DDR for older systems like athalons and pentium 4's
They also run at different voltages (just to make life simple right?) essentialy just pick out a stick already installed in the machine to figure out which your computer already supports.

667 what?
The 667 is the external clock speed in MHz, you want to match this as closely as possible to your processors "Front Side Bus" to keep things syncronised nicely, slapping in a faster module can have a detrimental effect because it complicates the external buss speed syncronisation timings.
Faster (ie bigger numbers) is better here, sync it to your FSB and your on to a winner.

PC5400
This is the maximum THEORETICAL transfer rate in MB/s so in theory a 2600 module (DDR) can transfer 2600megabytes every secons where as a 5400 (DDR2 module) can swap out 5400megabytes every second.

This number is calculated by timing the cycle frequency to the number of bits (8) so 667x8 = 5336mbps (it gets rounded up by the manufacturer to 5400, the same as hard drives are actualy smaller than quoted due to sloppy math)

3-4-4-8
LATENCY this is the single most important figure on a memory module, its also the most misunderstood and complicated and therefore very overlooked by average system builders.

Memory is a fairly complicated thing, it wont transfer a bit of memory on every cycle, it needs time to perform internal gubbins like refreshing things, reseting other things and so forth.

the first number is the CAS Latency (Column Address Strobe) this is how many cycles a memory moodule will take to return data to the processor, so 4 will take 4 clock cycles on the processor whereas a 2 only takes 2, when you consider that a 2ghz processor runs at 2,000,000,000 cycles every second, a memory module with a CAS of 4 will allow the processor to only run 500,000,000 (500MHz) of that for memory operations whereas a CAS of 2 will allow you to go at 1GHz (and computers work by fetching data from memory, performing a calcualation on that memory then sending it back to memory) so your computer performs 2 memory operatons to every calculation by their very design.

The second number it the tRCD (timed RAS to CAS Delay) its the time taken in clock cycles to move on to the RAS(row address strobe) from the CAS...
Memory is like a grid or spread sheet, bits are arranged into columns and rows, the column strob moves data long the horizontal columns the row strobe moves data up and down the rows. make sense? so the tRCD is the time it takes for the memory to stop moving along the bottom and start moving it up and down to get to the data. needless to say lower is better.

The next number is the tRP (timed RAS Precharge) the time it takes to stop fetching from one row to moving to another row. lower = better.

The final number is the tRAS (Active to Precharge Delay) this is how long it takes between charges, or how long it takes between the processor requesting one transfer to it requesting another.

So essentialy you want these numbers to be as low as you can get them within your budget.

Thats essentialy memory module timings explained in the least complicated way i could figure.
Rule of thumb match the DDR type to the supported motherboard type, match the first number to your processors FSB, ignore the third part, the string of digits on the end should be as LOW as you can get them.