With all due respect, you won't really get any form of 'future' proofing. Future proofing realistically is impossible, even harder on a LGA 775 socket. It is a very outdated socket, with outdated compatibility. The best types of processors that you can take on that board would be a Q9650 for example, but you're already above your $60 mark there. Equally the Q8200 I believe is above your $60 mark, so from my understanding, you can only downgrade.
Sorry to hold the pessimistic viewpoint! But realistically, my understanding is that from your budget, you can only downgrade, future proofing is technically impossible anyway, and the socket which is up to 10 years+ old now severely limits your capabilities realistically.
That is unless you can find a decently priced used CPU, the Q9650 i.e. But even then, that won't give you much for future usage.