shouldn't be all that difficult. really most hard drives should work fine with your current setup. all you will need to do is open the case, hook it up on the second connector of your ide cable, change current drive to master (using the jumper on the back of the drive), set the new drive to slave (also using jumper), plug up the power, and away you go. if you have windows xp professional, should be exceptionally easy.
might i recommend this <A HREF="http://www.newegg.com/app/viewProductDesc.asp?description=22-148-022&depa=1" target="_new"> Seagate 120gb Drive</A>? it should do just fine.
the only way you could get into trouble is if your current rig doesn't support a large drive. but messing with dell's insane support site should give you the information you need.