Join a pool or create my own pool with my 500 PCs?
With that much hash power, I believe you could be better off creating your own mini-pool and putting those computers in it. In theory, solo mining will always give you more coins in the long term for pools always take a fee for their services. In practice, of course, you could be mining for years without ever finding a block, so it's more a matter of how much you can wait to see a return on your costs and investment: more constant payments but less gain in the long term, or the inverse.

This formula can aproximate how much time on average would elapse before you found a block - it depended on the difficulty level of the network and your hashing power. Have a look at it and decide if you can wait that much weeks/months with no yield in order to gain more in the end.

https://en.bitcoin.it/wiki/Difficulty#How_soon_might_I_expect_to_generate_a_block.3F