The cheapest way to buy a drive is to buy oem, that is, a bare drive. This is the kind of drive you will get from New Egg or Other World Computing and most other online stores. If you buy brick-and-mortar, such as Best Buy, you'll get a boxed drive with
a cable, screws, some (windows) software, and a printed manual - and usually pay significantly more unless you get the unit on sale. You will almost certainly
have to format the drive to make it Mac compatible, but this is quite easy using Disk Utility. You can use both PATA and SATA drives in your machine, but would have to purchase a SATA pci card to do so, so that's probably out if price is important. I love Western Digital drives in G4s, but I've had good success with Hitachi in my G4. My recommendation would be at least 250g, such as this Western Digital:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822144701 - just $70. I don' t like the latest generation of Seagates as they draw a lot of power on startup and cause problems in some systems. Samsung makes great drives - cool running, quiet, fast - but have a slow access time so make a better backup drive than boot drive. Many people love Maxtors in their macs, but certain models run much hotter than other drives, so I avoid them also. If you are still using the original drive, almost any new drive will give you a noticeable boost, so don't sweat it too much. Good Luck.
mac pro, dual 1.25 g4 - Pioneer 111d, LG H10, Plextor 716UF, LG 4163,Pioneer 112 Mac OS X (10.4.9) dell 9200, dell xps 400 Benq 1650, Plextor Premium