I'd like to add more memory to my 5yr old Dell Dimension 2400.

I've currently got 2x256Mb in DDR1 and DDR2 (512Mb total)  and my owners-manual suggests a maximum of 1Gb  (512Mb in each slot).   But, is that just an old maximum dictated by how memory was available 5yrs ago?  Can I actually put a 1Gb DDR into each slot and give it 2Gb in total?   Or is there really a limitation due to the motherboard etc?    Some websites selling memory give conflicting opinions on this.   Thanks.   John