RAM is cheap these days so it won't make much difference, if any, to the secondhand sale price.
Same for hard drives - a 40MB drive these days is around £35-£45, an 80GB about £55-£65 and a 120GB around ££80 to £85. 17in monitors cost from about £85 upwards (all these prices are for new products).
A rewriter is about £30 upwards depending on make, a DVD-ROM drive (my Pioneer 116 cost more than £50 at the time) from £25 and a floppy costs under a fiver.
You don't list the CPU but I don't think that your system is worth what I would guess you are expecting...:-(