32bit Windows?
Here's what I think is happening:
> It recognises the first 2GB DIMM
> It recognises the second 2GB DIMM but if it adds it in, it would become 4GB which is not supported (remember 4GB is actually 4094MB or something).
> refuses to go further than that so the rest of the 512MB DIMMs are also ignored.
Anyway dude, even if you do manage some tricks, it will still give you just 1 more GB. Better upgrade to 64bit Windows, or ditch it altogether...