If you look on the post screen you will see the total memory figure exceed the usual amount listed when looking at the 1.024mb amount seen in Windows. The 1,024mb is actually the retailers rounded figure rather then the amount you don't see taken and reserved by the board itself for things like caches and basic hardware processes performed by the ide and other controllers.
Windows then takes some more for it's own background services as well as reserving a certain amount of memory addresses into higher memory. Vista does like a little more then XP there for the latest whatevers. The conversion from bytes into kilobytes and then into megabytes does lose something to end up with the rounded 512mb figure. To really see how this is done you need a math calculator like the one seen at
http://familyinternet.about.com/gi/...=http://www.onlineconversion.com/computer.htm
1024 Byte = 0.0009766 Megabyte (MB)
512 Megabyte (MB) = 524,288 Kilobyte (KB)
1024 Megabyte (MB) = 1,048,576 Kilobyte (KB)
2048 Megabyte (MB) = 2,097,152 Kilobyte (KB)
2 Gigabyte (GB) = 2,097,152 Kilobyte (KB)
2 Gigabyte (GB) = 2,048 Megabyte (MB)
2 Gigabyte (GB) = 2,147,483,648 Byte