RAM Upgrade: How do I tell if my PC supports 2700 vs 2100 AND is it better to use a pair of 512 or a single 1 GB RAM?

I wanted to upgrade my RAM in my Sony Vaio PCV RS 411 desktop (comes with 256MBx2 of RAM).  Like I have done in the past, I went out and used the system scanner/advisor tool on crucial.com.  It said I had 4 slots with a max of 2GB and recommeded some PC2700 RAM.  I wanted it quick so I went to BestBuy and bought a single Kingston module (1GB, 333Mhz, PC2700, DDR, CL2.5).  When I got home, I actually only had 2 slots (already filled).  The original RAM was 256MB of PC2100 266Mhz.  I replaced one of them with the new 1GB module I bought.  So, now I have a total of 1.25GB, but mixed of 2100 and 2700.  I've read that the lower of the two modules will be used.  I also later saw that my PC will only support 1GB max of RAM (even though all 1.25 are showing up when I look at the My Computer properties).  So, here are my questions.

1.  If my PC came with PC2100 RAM, how can I tell if it supports PC2700 RAM?  I added more information about my desktop below.

2.  Assuming 1GB maximum, is it better to use a pair of 512MB modules or a single 1GB module?  Does it matter if the 1GB is 2700 and the 512MB are 2100, or vice versa?

Thanks for you help.
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Gary CaseRetiredCommented:
Okay ... let's do a bit of a test ==>

Sony's site has limited documentation for this (a specs sheet and basic user guide), so it's a guess whether or not your system support dual channel mode.   I agree with Callandor that it probably does not - but you may as well know for sure.   So do this:

(1)  Install the two original memory modules that were in it.

(2)  If you haven't already (per Callandor's suggestion), download, install, and run Everest Home Edition.

(3)  Look at Motherboard - Chipset (on the left panel) and see what it shows for Memory Controller on the right => both "Type" and "Active Mode"

IF the Active Mode is Dual Channel (with the two original modules installed) then your system supports dual channel.   In that case, you'll get the best performance with two matched modules.

If the system does NOT support dual channel, then you should use a single stick of 1GB RAM instead of two sticks of 512mb => it not only gives you better future upgrade potential, but also is less of an electrical load on the memory controller (which can give you a slight reliability edge).

Don't worry about the PC2100 vs PC2700 mix => the PC2700 will simply run at the slower speed of the other module.
You can try it with just the PC2700 RAM installed and see if it boots.  Alternatively, get Everest and look under the motherboard section for the FSB setting: http://www.majorgeeks.com/download4181.html  PC2700 runs on a 166 motherboard FSB.

For pure speed, two matched modules running in dual channel mode is best, but I doubt your motherboard supports dual channel if it only has two slots.  In that case, one module is better for future expansion.  Since you are mixing two different speed and two different size modules, the whole system will run at the slower speed, and it doesn't matter which is which.
The bus speed on that board is 533 MHz, i.e. quad-pumped 133 MHz.  So, you'd want to run PC2100 (266 MHz, double-pumped 133 MHz) so that everything is using the same base timings (133 MHz).  The PC2700, if coupled with a PC2100 module, will throttle back to 266 MHz.  Do you overclock?
dhsindyRetired considering supplemental income.Commented:
Good advice/information above.

One piece of advice - get the proper RAM and install the maximum amount as soon as you can afford it.  It will only get more expensive.  On an old PC that had EDO ram I waited to long to upgrade - the available supply diminished and the price went through the roof.
If the memory is showing up in Windows, it's being supported. Whether it's being supported reliably or not is another story. You'll want to run a memory test, such as memtest86+ (http://www.memtest.org/). If you get errors, run it again with the original memory. If not, you're pretty much good to go. The documentation for your computer may have been written before 1 GB chips were common, so the most that's officially supported, and tested as working, was two 512 MB chips.

Whether your system supports dual channel or not, 1.25 GB of single channel will be much faster than 256 MB of dual channel.

As far as the speed's concerned, garycase is correct. The speed on a memory chip is the *maximum* speed at which it is rated. Putting RAM rated faster than your system's bus speed is like driving a sports car on a Los Angeles freeway in rush hour. The car will work just fine, but it might be overkill. If you can get the faster RAM for the same price, or even less than, the slower RAM, get the faster RAM. On the other hand, don't spend twice as much for memory that's faster than what you need.

http://www.crucial.com/crucial/pvtcontent/memorytype.asp?model=&memtype=CHOOSE#speed confirms this by the way, which says, "it's okay to mix the speed of the memory in a system. Just keep in mind that your computer is made to handle a specific memory speed, so even if you add a faster speed memory, your computer will only allow it to run at the speed your computer was made to handle."
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