The new replacement drives for IDE are called SATA drives which I use. The big cache and very fast access times beat the IDE drives some 3 to 4 times and that is an improvement of major prorportions without spending the big bucks. You do need a mother board that will accept them however.
Also the FSB speed is important. What good is fast paired memory and fast drives if you can't move the data. Intel has one mother board with 1024 FSB (correct me if I got the speed wrong) and the board I got has an FSB (Front Side Bus) of 800 (I could not get the faster one).
Most home come computers (1.0 / 2.0 MHz) are 400 FSB. So think about that befor going headlong in getting new drives etc.
There is a serious discussion going on right now IF there is any benifit using DDR II 512 paired ram over regular ram - sorry it is waaaaay over my head. Just passing the info on. My 2 gig (DDR II 512) cost me $600.00 last December - something else to think about.