From the manual: DDR2 PC2-6400 with a maximum of 4GB per slot.
You have two SODIMM slots, so you can install up to 8GB.
Edit: let me extend the answer a bit since there is a lot of confusion surrounding this:
There are a couple of XY bit parts in a CPU.
- How does the CPU work internally. How wide are its registers? (usually 32 or 64bit)
- How wide is the data path to the memory
- How wide is the address space
Most people use item 1 to describe the 'bittiness' of the CPU.
E.g. take an old motorola 68000 CPU. Its internal workings are 32 bits and it is referred to as a 32bit CPU. Even though it has a 16 bit path to the memory and 24 bit of address space. This means it could only access up to 16MB of memory.
The C2D CPU found in the E6500 can do either 64bits internally, or it can be left in a mode where it uses 32bits. Your OS needs to match this. (Read: it boots in the old 32 bit mode and windows 64 will enable the amd64 functionality somewhere during boot).
Calculating with 32 bits internally does not mean the path to the memory is also 32 bit. Most modern CPU support a stopgap measure called Physical address extension. This is supported by windows. As a result even a 32 bit version of windows XP/vista/7 can access much more than 4GB.
The caveeat is two fold:
- Your drivers need to understand this. Not all do. This is not a problem when installing a server OS with standard server hardware, but finding stable PAE aware drivers for consumer hardware can be a challenge.
- Your programs are still limited to 2GB virtual memory (address space). There is no limit (other than the OS limit) on how much physical memory (RAM) they can use.
If you plan to use lots of memory (e.g. using VMs, each with <= 2GB memory) or if you plan to upgrade the laptop to a 64bit OS then it it worth buying the memory now while it still is available in shops.