Jazz,
Do you know, have you a separate graphics card or is it an onboard embedded card?
If you have an onboard graphics card then get a couple of 1GB cards for slots 3 and 4, leave the 2 x 256 MB in and that way you will have plenty of memory after your graphics have laid claim to their bit of memory from your RAM.
If you have aseparate graphics card the the two 512 MB cards would be enough along with your existing 2 x 256 MB. That would give you about 1½ GB for your RAM.
dave