Founder is fine, co-founder is too. Co-founder automatically conveys that you are not a single founder, with just 'founder' someone might infer that you were either first or the only founder. It doesn't matter much in practice, either is fine. It distinguishes you from a regular hire and it is clear that you can be addressed with any issues.
I am about to start a "Sole Proprietorship" to do some VoIP consulting and system integration. I have had 2 clients before this, but I was just getting my foot in, so I didn't have a "Company Name". I am glad this came up on HN. Would 'Founder' be the most appropriate title on my biz card?
For a one-man shop, I'd just skip the title entirely. The only downside is potentially someone seeing just the card wouldn't know if you have the authority to do deals, but I really doubt that would be an issue with a consulting shop.
