I want to get my sister and I a new hard drive because there is very little space on it. Either a 160 or 320 gb hard drive. I was looking around on newegg and found one that i thought would have worked well but one of the reviews said it should just be used as a storage drive because it only had 8mb cache which led to a slower start up. This computer is running xp and i wasnt sure if a hdd with 16mb cache would perform that much better than one with 8mb.
Slightly better, yes, and the ones with 32 MB caches just a little better again. The differences are in the time to locate a file and feed its data to you. If you don't need speed you probably can tolerate the difference easily. A larger cache is really a help at working with access to long sequential files, but less help when your application has to jump all over the disk looking for new small files. Gamers love big caches, fast HDD rotational speeds, RAID0 arrays - anything that gives a tiny margin of improved performance. Many everyday users find these small improvements hard to notice.
I used a 8MB to install a game demo stored on the drive. I think the demo was either call of duty 4 or unreal tournament 3 or both and it took forever compared to a 16MB. Never used a 32MB but sounds fast. Try to experiment with those mentioned demos and get out your stop watch. Otherwise if you keep your drive defragmented it doesn't really effect actual in game performance.