(apacer 512mb unb pc2-4300 cl4)

SOURCE: Problem with Mercury PI845GVM motherboard

fine ur problem if very simple as u installed 512 mb RAM on ur pc if at possible try to connect on to the other slot and restart the computer and check ur RAM capacity in ur os if it shows as same 256mb then ur have to change ur virtual memory settings i.e memory allocated to the video so configure ur virtual memory as per requirement then free up RAM memory ..

Steps for virtual memory configuration.

1. Open System in Control Panel.
2. On the Advanced tab, under Performance, click Settings.
3. On the Advanced tab, under Virtual memory, click Change.
4. Under Drive [Volume Label], click the drive that contains the paging file you want to change.
5. Under Paging file size for selected drive, click Custom size, and type a new paging file size in megabytes in the Initial size (MB) or Maximum size (MB) box, and then click Set. If you decrease the size of either the initial or maximum page file settings, you must restart your computer to see the effects of those changes. Increases typically do not require a restart.
   

SOURCE: RAM not found as 2x512MB

Are you finding these number when you go into your bios or from a program such as CPU-Z? If you are using good quality DDR400 memory, it should be posting with the full amount of memory available. You are using the correct slots to get dual channel, but you may want to try slots 1 & 2 or 3 & 4 to see if it will post with the full amount of memory as single channel. You may also want to have a friend try the memory in their computer to see how it posts, you may have some bad memory.

SOURCE: Added 1Gb RAM DDR2 533 PC2 4300 CL5 but recognizes only 512Mb

the is a BIOS problem. you can still google search the BIOS updater. OR have a look on windows how much it is being detected.