Creating new instance of SHA1CryptoServiceProvider every time you need to compute a hash is NOT fast at all. Using the same instance is pretty fast.
Still I'd rather do one of the many CRC algorithms instead of a cryptographic hash as hash functions designed for cryptography don't work too well for very small hash sizes (32 bit) which is what you want for your GetHash() override (assuming that's what you want).
Check this link out for one example of computing CRC in C#: http://sanity-free.org/134/standard_crc_16_in_csharp.html
P.S. the reason you want your hash to be small (16 or 32 bit) is so you can compare them FAST (that was the whole point of having hashes, remember?). Having hash represented by a 256-bit long value encoded as string is pretty insane in terms of performance.
