Assuming you are running Windows, I'd get a 120-250GB. They are relatively inexpensive. Most programs want to install to the boot drive and some do not give you an option to relocate to another drive/partition. A drive in that range of size will give you plenty of room and, if you also have a second or external drive, you can move data to those.

As far as brands of hard drives, I tend to prefer Seagate and Western Digital.

I've got a 128 GB Crucial SSD at home.  I keep all my data on another drive.  At work I'm running two 128 GB Crucial SSDs in a RAID 0 configuration (256 GB boot drive), and again have my data on another drive.  Both setups work very well for me.

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That question is just a bit too personal.

2-500GB in computer, and 300GB external for back-up. When I picked up teh external, I had a 150GB inside so it was plenty, now I'm close to needing another back-up drive, or one to dedicate to classic porn.

I have a 250GB SSD for my boot C: and a 1TB drive which is partitioned for 95% into a data drive (D: and mounted onto C:\Data) and the rest into a temp drive.

I have my swap file, temp directories, downloads, and browser cache set up on my temp drive which I have mounted into C:\Temp.  This is to reduce activity on the SSD which can lesson it's life span.

Most apps are installed on D: unless they are small in size.

After initial install of Win7 64bit Ultimate and drive config and auto-login enabled, it took 9 seconds to go from boot screen (win logo) to desktop with icons.

C: 140,24 GB (only used for OS and software)
D: 913 GB

Both are Western Digital. Never had one fail o me so far (knocking on wood)

Internal drive: Samsung, SATA, 149 GB, split into 50 GB on C:, 99 GB on D:
External drive 1: Western Digital "My Book", USB, 913 GB on H:
External drive 2: Seagate Expansion, USB, 1.81 TB on I:

Quoting Dr. Killpatient:

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Quoting Taro:

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Both are Western Digital. Never had one fail o me so far (knocking on wood)

I have a knack for killing hard drives within a certain proximity.  There's a stack of bad drives sitting next to my desk with many brands and most that died less than one year from manufacture.  Seems that drives within arm's reach have their life span cut.

If you are a heavy user try the professional series of Western Digital (RE3 (Heavy-Duty with a MTBF of 600.000 Start/Stop-cycles compared to 300.000 on a "normal" retail drive) or RE4 (Duty Type S1 (always on) with a MTBF of 1.2 Mio hours)

I use them for Servers, NASs and professionally used PCs. Failure rate is extremely low here, the price is about three times higher than the normal retail version though.