a program such as partition magic should allow you to shrink your existing partition down to the 74GB required (thats the unformatted amount though), formatted with NTFS, itll have to be shrunk down to 68GB (which is still more than is needed for an os hdd, if youre only storing the os and applications on it)
as far as performance difference between it, and a marginally slower, yet much larger (current) 7200rpm drive... there isnt a whole lot, to be honest... with a raptor, youll be able to access files quicker, and load them quicker, and boot into windows quicker, but again, it will only be by just a few seconds on average (making your os that much more responsive as well)...
if youre more concerned about the price/performance/capacity ratio though (compared to other consumer hdds)... raptors as a whole are way outmatched... a more ideal hdd in that case would be a hdd such as a seagate 320GB 7200.10 sata hdd, for <$100... its closer to the performance of the older 36GB/74GB 8MB GD series raptors, which is still fast nonetheless, but offers much greater capacity
not to mention the seagate runs cooler and is quieter as well, primarily due to the RPM difference (10k to 7.2k)