Which computer is faster 160 GB or 250 GB RPM

The total space on your computer isn't the key issue, although the more junk like lots of unused program files will slow you down.

You need to look at RAM , 2GB RAM should be ok for everyday use, 4GB ram is probably the highest you could get in a high street store, (for now at least, wouldn't be suprised if there was a 16 or 32 GB ram before 2020)