No, XP Home is limited to 2GB though there might be a registry hack to expand it like there is with Pro. If there is, I haven't heard of it.
XP Pro is limited to 3GB although a reg-hack will make it accept 4GB. Doing this results in minimal gain (it doesn't fully utilize it) with the cost of system instability.
This is not correct.
Based on the success of several others using XP Home with 4 GB of RAM and the 3 GB switch (reported in other posts), and an e-mail reply from a Microsoft MVP indicating that XP Home and Pro both support this switch I purchased 2 GB more RAM. I am now successfully using XP Home with this switch and the 'fine tuning' switch userva to achieve system stability with an extra 1 GB of virtual memory available for applications (that support these switches).
According to Adobe, Photoshop CS2 and CS3 can address this extra virtual RAM (not CS, or earlier I believe. See the Adobe Photoshop "TechNotes" on optimization.) It is my understanding that the 3 GB switch is not guaranteed to work on all computers, depends on chipset and other things apparently?
Details can be found in the following thread in the DP Review PC Talk forum: