Your roi will increase based upon your cash you have in the property but your risk will increase. Only you can answer that question as to what is best for you.
Generally banks will consider doing a cash out refinance after 12 months of ownership based upon the current appraised value. A cash out refinance on an investment property will typically be capped at 75% LTV.
If you are wanting to acquire more rentals and feel that you can acquire more similar to this than adding leverage sounds like it might work for you. As far as how much cash should you take out, I would suggest you take as much as possible when you do the refinance as you will pay closing costs and would want to take advantage of financing as much as possible on 1 property that you can.
As someone mentioned one issue is many lenders don't like to do these small loan amounts and then you have consider the closing costs will most likely be several thousand dollars to borrow the money. If closing costs are $2,500 and you borrow $25,000, it is costing you 10% upfront to borrow the money. Probably still worth doing if you want to and are able to continue buying more properties just like this one.
I will be looking at similar situation for a property I paid $35,000 cash for at the end of June 2012, hoping to do cash out to take all of my money back out of the property in July of this year.