If you don't plan on the jump to a 64 bit operating system, don't bother with 4GB of Ram. A 32 bit operating system will not ( read that cannot ) take advantage of it. On a related note, many motherboards will drop your clock speed down if you fully load it up with memory sticks. Keep it in mind.

The ONLY way Windows can see 3GB or Ram or more on a 32 bit operating system is via the use of the /3GB flag in your boot.ini file. Even THEN, only those programs written to actually utilize that much RAM will be able to take advantage of it.

Bottom line, if you're not going to do a 64-bit OS, don't bother with taking your Ram past the 2GB mark ( 3GB max IF your motherboard doesn't whack the FSB after loading up on ram sticks )