You'll pay about 25% more for a 7,200rpm drive, they are the ultimate to get if you "feel the need for speed"
It really depends on what you want from your system, If your aim is the fastest, both graphically and work intensively then you have to get a 7,200, on the other hand if space is more of a priority than speed the differance in cost between a 7,200 and a 5,400 could well get you an extra 10 gig but with a slightly slower drive.
ie: 20 gig 7,200's are about the same price as a 30 gig 5,400.
As for ATA 100. as was said earlier, most are now 100, and are backward compatible so don't settle for less, although hard drive data transfer has not yet reached the old UDMA 66 rate yet, so it may on the face of it look pointless to have UDMA 100, but I am sure these rates will rise as quicker board chipsets are introduced in the near future.
Martin