'have 2x 4GB memory sticks DDR2 but my computer is only reading 2.50GB but when i put 1x 2GB and a 1x 1GB it reads 3GB '

'I am running 32-bit windows 7 '

The 4gb virtual memory address limit for 32 bit operating systems.
An example of 3gb working better than 4gb in a 32 bit operating system.

See Response 6:
http://www.computing.net/answers/ha...

You often have more ram available for the user and Windows when you install 3gb rather than 4gb or more.
.....

You may have not have any programs that benefit from more than 3gb of ram.

Memory Assessor
How much memory do you really need?
http://www.kingston.com/tools/asses...
........

My laptop came installed with a 4GB RAM in slot2. The other slot has been free. Today I bought another 4GB module with same specifications and after inserting to slot1, it did not get detected. I swapped the modules, and the new RAM is functioning perfectly. However, the RAM in slot1 is not getting. Memory slot not reading ram. This is a one roll bet. Don't have a Kindle?The gradual introduction of video slots which the company has become seromucous for has helped secure their status as a leading developer.We offer traditional paper and electronic bingo units. Another benefit of playing free casino slots games at Slotomania are our special perks: from free spins to bonus rounds, making your game even more enthralling. This product memory slot not reading ram is intended for use by those 21 or older for amusement memory slot not reading ram purposes only.

Memory Slots In Laptop

A better explanation I've seen.

Memory Slots Used

Memory Slot Not Reading Ram Free

If you have a 32 bit operating system and you have installed 4gb (or more) in the mboard, the amount of that ram available to Windows and the user is
4gb minus the amount of ram your devices require or have.
E.g.
- minus the amount of ram shared with onboard video if you're using that
- and/or - minus the amount of ram your dedicated video card installed in a slot has.
If you have more than one video card installed in a slot, it would be minus the ram for all of them.
(and = Some mboard main chipsets have Hybrid video capability. If you have a PCI-E X16 video card installed in a mboard slot that is supported by that feature, it's often the default for both the onboard video and the video on the card to work at the same time, unless you change default bios Setup settings. In that case it would be minus the ram for both of them.)
- minus the amount of ram your sound adapter has, if that applies - e.g. sound cards installed in a mboard slot often have ram
- minus the amount of ram any other devices have, other than hard drives and optical drives, if that applies.