Mac – How to know if I need a new hard drive

Doesn't sound like a hard drive failure to me; maybe RAM or - worse - anything logic board.

Try the already mentioned AHT and try to find error logs via Console (if you don't know where to start looking, try narrowing your options by looking at the timestamps, they'll pretty much stop after whatever went wrong and then continue after the reboot with a noticeable time shift)

Other things you could try:

• Verifying Volume and File Permissions, ideally while booting from a recovery/install disc, not the installation on the hard drive.While the graphical errors don't seem to be caused by a hard disk problem, the OS crashes themselves might be because of corrupted files that cause problems when being read/written.
• Running a memory test and / or removing RAM modules to see whether one of those is faulty. In my experience though, faulty memory is more likely to result in the device not starting up at all.
• Try to check the temperatures of various parts of your machine, e.g. with iStats (and/or check whether you can simply feel your machine getting noticeably hot). Graphical errors and/or freezes might be due to overheating of components resulting in an emergency shutdown to prevent damage to the hardware.

"I would like to know if hard drive access speed is a bottleneck to my system currently."

Of course! There are two aspects to a disk storage system:

1. Bus speed
2. Storage medium speed

On ANY given bus, e.g., SATA, SATA2, SATA3, a mechanical HDD will be an order of magnitude slower than an SSD for a given link speed. The secret is to always use an SSD that features at minimum the same link speed as the disk controller. It can be of benefit, however, to use an SSD (or HDD if you're financially challenged) that features a faster link speed than its host.

For example, I'm using a Crucial M500 960 GB SSD in my MacBook4,1. The SSD is a SATA 3, 6 Gb/s drive, whereas the MacBook has a SATA 1.5 Gb/sec controller. The drive performance is marginally (!) better due to its higher internal processing speeds than would be a 1.5 Gb/sec SATA SSD on the same system.

Mechanical hard disks are painfully slow compared to their SSD counterparts. In ANY computer system, the HDD and its attendant controller will be the biggest bottleneck. As such, to ensure best performance, you should use the fastest storage medium possible.