There is a 3GB ram address space limit in 32bit operating systems. The last GB is used for other uses on the system. The way this was displayed was changed in Vista and "fixed" in SP1.
If you want more than 3GB, you need to use a 64bit version of windows.
There are more details here http://www.dansdata.com/askdan00015.htm and this microsoft support KB article
If you are running Windows Vista or if
you have upgraded Windows XP to
Windows XP Service Pack 2 (SP2) or a
later version, the memory that is
reported in the System Properties
dialog box or by the System
Information tool (Msinfo32.exe) has
changed. This symptom occurs if the
following conditions are true:
- The computer has 4 or more gigabytes (GB) of RAM.
- The computer is using Physical Address Extension (PAE) mode.
For example, before you upgrade to
Windows XP SP2, the System Properties
dialog box may report approximately
3.87 GB of RAM.
The System Information tool may report that the total
physical memory is approximately
3,540.00 megabytes (MB). After you
upgrade to Windows XP SP2, the System
Properties dialog box may report
approximately 3.12 GB of RAM, and the
System Information tool may report
that the total physical memory is
approximately 2,770.00 MB.
This issue occurs because of a design
change in Windows XP SP2 that is also
included in Windows Vista. The changes
were made to PAE mode behavior to
improve driver compatibility.
To reduce driver compatibility issues,
Windows Vista and Windows XP Service
Pack 2 or a later version include
hardware abstraction layer (HAL)
changes that mimic the 32-bit HAL DMA
behavior. The modified HAL grants
unlimited map registers when the
computer is running in PAE mode.
Additionally, the kernel memory
manager ignores any physical address
that is more than 4 GB. Any system RAM
that is more than the 4 GB barrier
would be made unaddressable by Windows
and be unusable in the system. By
limiting the address space to 4 GB,
devices with 32-bit DMA bus master
capability will not see a transaction
with an address that is more than the
4 GB barrier. Because these changes
remove the need to double-buffer the
transactions, they avoid a class of
bugs in some drivers that is related
to the correct implementation of
double buffering support.
Also application launch times are more about disk IO speed than the amount of RAM you have, hence why eclipse is no faster to load.