Whats the difference in cost between speccing your own set up and just buying one already made as a package?

Cos want to replace this machine some time soon as it is well dated, has tiny HDD and not a very good processor....

Currently running Win 98, think it's 256mb RAM, 9.summat gig HDD, not sure on preocessor, usual windows stuff on it (word, excell all that shit) and mainly used for 512 ADSL net surfing, tho my old dear does use it for school work (teacher)

other hardware is : CDRW E drive, DVDRW D drive, normal 244mb floppy A drive, along with a multimedia card reader/writer (smart mediam, multi media, memory stick etc)

So whats good and whats not? and what cost am I talking? Not fussed about looks really, but a decent screen size would be nice (flat TFT sounds cool )